JavaScript Coding Practice Questions

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Best 300 JavaScript Coding Practice Questions


Q. Write a program in javascript. sum(2)(3); // Expected output is 5

function sum(x, y) {
  if (y !== undefined) {
    return x + y;
  } else {
    return function (y) {
      return x + y;
    };
  }
}

Output

console.log(sum(2,3));   // Outputs 5
console.log(sum(2)(3));  // Outputs 5

Q. How to validate file size and extension before upload?

<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
    <title>Show File Data</title>
    <script type="text/javascript">
      function showFileSize() {
        var input, file, extension;

        // (Can't use `typeof FileReader === "function"` because apparently
        // it comes back as "object" on some browsers. So just see if it's there
        // at all.)
        if (!window.FileReader) {
          bodyAppend("p", "The file API isn't supported on this browser yet.");
          return;
        }

        input = document.getElementById("fileinput");
        if (!input) {
          bodyAppend("p", "Um, couldn't find the fileinput element.");
        } else if (!input.files) {
          bodyAppend(
            "p",
            "This browser doesn't seem to support the `files` property of file inputs."
          );
        } else if (!input.files[0]) {
          bodyAppend("p", "Please select a file before clicking 'Load'");
        } else {
          file = input.files[0];
          extension = file.name.substring(file.name.lastIndexOf(".") + 1);
          bodyAppend(
            "p",
            "File Name: " +
              file.name +
              "<br/>File Size: " +
              file.size +
              " bytes <br/>File Extension: " +
              extension
          );
        }
      }

      function bodyAppend(tagName, innerHTML) {
        var elm;

        elm = document.createElement(tagName);
        elm.innerHTML = innerHTML;
        document.body.appendChild(elm);
      }
    </script>
  </head>
  <body>
    <form action="#" onsubmit="return false;">
      <input type="file" id="fileinput" />
      <input
        type="button"
        id="btnLoad"
        value="Load"
        onclick="showFileSize();"
      />
    </form>
  </body>
</html>

Q. Create captcha using javascript?

<!DOCTYPE html>
<html>
  <head>
    <title>JavaScript Captcha Example</title>
  </head>
  <script>
    var captcha;

    function generateCaptcha() {
      var a = Math.floor(Math.random() * 10);
      var b = Math.floor(Math.random() * 10);
      var c = Math.floor(Math.random() * 10);
      var d = Math.floor(Math.random() * 10);

      captcha = a.toString() + b.toString() + c.toString() + d.toString();
      document.getElementById("captcha").value = captcha;
    }

    function check() {
      var input = document.getElementById("inputText").value;

      if (input == captcha) {
        alert("Valid Captcha");
      } else {
        alert("Invalid Captcha");
      }
    }
  </script>
  <body onload="generateCaptcha()">
    <input type="text" id="captcha" disabled /><br /><br />
    <input type="text" id="inputText" /><br /><br />
    <button onclick="generateCaptcha()">Refresh</button>
    <button onclick="check()">Submit</button>
  </body>
</html>

Q. Create a Stopwatch program in javascript?

<!DOCTYPE html>
<html>
  <head>
    <title>Stopwatch Example</title>
  </head>
  <body>
    <form action="" method="post">
      <h4>Simple stopwatch made in JavaScript</h4>
      <input type="button" onclick="startWatch()" value="START" />
      <input type="button" onclick="stopWatch()" value="STOP" />
      <input type="button" onclick="resetWatch()" value="ZERO" />
    </form>
    <p id="res">
      <span id="min">0</span> : <span id="sec">00</span> :
      <span id="msec">000</span>
    </p>
    <p>
      In this example Date() methods co-operate with timing function
      setInterval().
    </p>

    <script type="text/javascript">
      var timer = null;
      var min_txt = document.getElementById("min");
      var min = Number(min_txt.innerHTML);
      var sec_txt = document.getElementById("sec");
      var sec = Number(sec_txt.innerHTML);
      var msec_txt = document.getElementById("msec");
      var msec = Number(msec_txt.innerHTML);
      function stopTimeMilliseconds(timer) {
        if (timer) {
          clearInterval(timer);
          return timer;
        } else return timer;
      }
      function startTimeMilliseconds() {
        var currDate = new Date();
        return currDate.getTime();
      }
      function getElapsedTimeMilliseconds(startMilliseconds) {
        if (startMilliseconds > 0) {
          var currDate = new Date();
          elapsedMilliseconds = currDate.getTime() - startMilliseconds;
          return elapsedMilliseconds;
        } else {
          return (elapsedMilliseconds = 0);
        }
      }
      function startWatch() {
        // START TIMER
        timer = stopTimeMilliseconds(timer);
        var startMilliseconds = startTimeMilliseconds();
        timer = setInterval(function () {
          var elapsedMilliseconds = getElapsedTimeMilliseconds(
            startMilliseconds
          );
          if (msec < 10) {
            msec_txt.innerHTML = "00" + msec;
          } else if (msec < 100) {
            msec_txt.innerHTML = "0" + msec;
          } else {
            msec_txt.innerHTML = msec;
          }
          if (sec < 10) {
            sec_txt.innerHTML = "0" + sec;
          } else {
            sec_txt.innerHTML = sec;
          }
          min_txt.innerHTML = min;
          msec = elapsedMilliseconds;
          if (min >= 59 && sec >= 59 && msec > 900) {
            timer = stopTimeMilliseconds(timer);
            return true;
          }
          if (sec > 59) {
            sec = 0;
            min++;
          }
          if (msec > 999) {
            msec = 0;
            sec++;
            startWatch();
          }
        }, 10);
      }
      function stopWatch() {
        // STOP TIMER
        timer = stopTimeMilliseconds(timer);
        return true;
      }
      function resetWatch() {
        // REZERO TIMER
        timer = stopTimeMilliseconds(timer);
        msec_txt.innerHTML = "000";
        msec = 0;
        sec_txt.innerHTML = "00";
        sec = 0;
        min_txt.innerHTML = "0";
        min = 0;
        return true;
      }
    </script>
  </body>
</html>

Q. Write a program to reverse a string?

function reverseString(str) {
  let stringRev = "";
  for (let i = str.length; i >= 0; i--) {
    stringRev = stringRev + str.charAt(i);
  }
  return stringRev;
}
alert(reverseString("Pradeep")); // Output: peedarP

Q. How to check if object is empty or not in javaScript?

function isEmpty(obj) {
  return Object.keys(obj).length === 0;
}

Q. JavaScript Regular Expression to validate Email

var pattern = /^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/;

Q. Use RegEx to test password strength in JavaScript?

var newPassword = "Pq5*@a{J";
var regularExpression = new RegExp(
  "^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*[!@#$%^&*])(?=.{8,})"
);

if (!regularExpression.test(newPassword)) {
  alert(
    "Password should contain atleast one number and one special character !"
  );
}
RegExDescription
^The password string will start this way
(?=.*[a-z])The string must contain at least 1 lowercase alphabetical character
(?=.*[A-Z])The string must contain at least 1 uppercase alphabetical character
(?=.*[0-9])The string must contain at least 1 numeric character
(?=.[!@#$%^&])The string must contain at least one special character, but we are escaping reserved RegEx characters to avoid conflict
(?=.{8,})The string must be eight characters or longer

Q. How to compare objects ES6?

Example 01:

const matches = (obj, source) =>
  Object.keys(source).every(
    (key) => obj.hasOwnProperty(key) && obj[key] === source[key]
  );

console.log(
  matches({ age: 25, hair: "long", beard: true }, { hair: "long", beard: true })
); // true
console.log(
  matches({ hair: "long", beard: true }, { age: 25, hair: "long", beard: true })
); // false
console.log(
  matches({ hair: "long", beard: true }, { age: 26, hair: "long", beard: true })
); // false

Example 02:

const k1 = { fruit: "🥝" };
const k2 = { fruit: "🥝" };

// Using JavaScript
JSON.stringify(k1) === JSON.stringify(k2); // true

Example 03:

const one = {
  fruit: "🥝",
  energy: "255kJ",
};

const two = {
  energy: "255kJ",
  fruit: "🥝",
};

// Using JavaScript
JSON.stringify(one) === JSON.stringify(two); // false

Q. How to remove array element based on object property?

var myArray = [
  { field: "id", operator: "eq" },
  { field: "cStatus", operator: "eq" },
  { field: "money", operator: "eq" },
];

myArray = myArray.filter(function (obj) {
  return obj.field !== "money";
});

Console.log(myArray);

Output

myArray = [
    {field: "id", operator: "eq"}
    {field: "cStatus", operator: "eq"}
]

Q. Predict the output of the following JavaScript code?

console.log(+"meow"); // Output: NaN

Q. Predict the output of the following JavaScript code?

var result;
for (var i = 5; i > 0; i--) {
  result = result + i;
}
console.log(result); // Output: NaN

Q. Predict the output of the following JavaScript code?

var a = 1.2;
console.log(typeof a); // Output: Number

Q. Predict the output of the following JavaScript code?

var x = 10;
if (x) {
  let x = 4;
}
console.log(x); // Output: 10

Q. Predict the output of the following JavaScript code?

console.log(0.1 + 0.2 == 0.3); // Output: false

Q. Predict the output of the following JavaScript code?

console.log(1 + -"1" + 2); // Output: 2

Q. Predict the output of the following JavaScript code?

(function (x) {
  return (function (y) {
    console.log(x);
  })(10);
})(20); // Output: 20

Q. Predict the output of the following JavaScript code?

var num = 20;
var getNumber = function () {
  console.log(num);
  var num = 10;
};
getNumber(); // Output: undefined

Q. Predict the output of the following JavaScript code?

function f1() {
  num = 10;
}
f1();
console.log("window.num: " + window.num); // output: 10

Q. Predict the output of the following JavaScript code?

console.log("(null + undefined): " + (null + undefined)); // Output: NaN

Q. Predict the output of the following JavaScript code?

(function () {
  var a = (b = 3);
})();

console.log("value of a : " + a); // Output: undefined
console.log("value of b : " + b); // Output: 3

Q. Predict the output of the following JavaScript code?

var y = 1;
if (function f() {}) {
  y += typeof f;
}
console.log(y); // Output: 1Object

Q. Predict the output of the following JavaScript code?

var k = 1;
if (1) {
  eval(function foo() {});
  k += typeof foo;
}
console.log(k); // Output: 1undefined

Q. Predict the output of the following JavaScript code?

var k = 1;
if (1) {
  function foo() {}
  k += typeof foo;
}
console.log(k); // Output: 1function

Q. Predict the output of the following JavaScript code?

console.log("(-1 / 0): " + -1 / 0); // Output: -Infinity
console.log("(1 / 0): " + 1 / 0); // Output: Infinity
console.log("(0 / 0): " + 0 / 0); // Output: NaN
console.log("(0 / 1): " + 0 / 1); // Output: 0

Q. Predict the output of the following JavaScript code?

var a = 4;
var b = "5";
var c = 6;

console.log("(a + b): " + (a + b)); // Output: 45
console.log("(a - b): " + (a - b)); // Output: -1
console.log("(a * b): " + a * b); // Output: 20
console.log("(a / b): " + a / b); // Output: 0.8
console.log("(a % b): " + (a % b)); // Output: 4

Q. Predict the output of the following JavaScript code?

console.log("MAX : " + Math.max(10, 2, NaN)); // Output: NaN
console.log("MAX : " + Math.max()); // Output: -Infinity

Q. Predict the output of the following JavaScript code?

(function () {
  var a = (b = 3);
})();

console.log("a defined? " + (typeof a !== "undefined")); // Output: true
console.log("b defined? " + (typeof b !== "undefined")); // Output: true

Q. Predict the output of the following JavaScript code?

var myObject = {
  foo: "bar",
  func: function () {
    var self = this;
    console.log("outer func:  this.foo = " + this.foo); // Output: this.foo = bar
    console.log("outer func:  self.foo = " + self.foo); // Output: self.foo = bar
    (function () {
      console.log("inner func:  this.foo = " + this.foo); // Output: this.foo = function foo() {}
      console.log("inner func:  self.foo = " + self.foo); // Output: self.foo = bar
    })();
  },
};
myObject.func();

Q. Predict the output of the following JavaScript code?

console.log(0.1 + 0.2); // Output: 0.30000000000000004
console.log(0.1 + 0.2 == 0.3); // Output: false

Q. Predict the output of the following JavaScript code?

(function () {
  console.log(1);
  setTimeout(function () {
    console.log(2);
  }, 1000);
  setTimeout(function () {
    console.log(3);
  }, 0);
  console.log(4);
})();
// Output: 1, 4, 3, 2

Q. Predict the output of the following JavaScript code?

var arr1 = "john".split("");
var arr2 = arr1.reverse();
var arr3 = "jones".split("");
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1)); //array 1: length=5 last=j,o,n,e,s
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1)); //array 2: length=5 last=j,o,n,e,s

Q. Predict the output of the following JavaScript code?

console.log(1 + "2" + "2"); // Output: 122
console.log(1 + +"2" + "2"); // Output: 32
console.log(1 + -"1" + "2"); // Output: 02
console.log(+"1" + "1" + "2"); // Output: 112
console.log("A" - "B" + "2"); // Output: NaN2
console.log("A" - "B" + 2); // Output: NaN

Q. Predict the output of the following JavaScript code?

for (var i = 0; i < 5; i++) {
  setTimeout(function () {
    console.log(i);
  }, i * 1000);
}
// Output: 145, 5, 5, 5, 5, 5

Q. Predict the output of the following JavaScript code?

for (var i = 0; i < 5; i++) {
  (function (x) {
    setTimeout(function () {
      console.log(x);
    }, x * 1000);
  })(i);
}
//Output:- 0, 1, 2, 3, 4

Q. Predict the output of the following JavaScript code?

console.log("0 || 1 = " + (0 || 1)); // Output: 1
console.log("1 || 2 = " + (1 || 2)); // Output: 1
console.log("0 && 1 = " + (0 && 1)); // Output: 0
console.log("1 && 2 = " + (1 && 2)); // Output: 2

Q. Predict the output of the following JavaScript code?

console.log(false == "0"); // Output: true
console.log(false === "0"); // Output: false

Q. Predict the output of the following JavaScript code?

var a = {},
  b = { key: "b" },
  c = { key: "c" };

a[b] = 123;
a[c] = 456;
console.log(a[b]); // Output: 456

Q. Predict the output of the following JavaScript code?

console.log(
  (function f(n) {
    return n > 1 ? n * f(n - 1) : n;
  })(10)
); // Output: 3628800

Q. Predict the output of the following JavaScript code?

(function (x) {
  return (function (y) {
    console.log(x); //1
  })(2);
})(1);

Q. Predict the output of the following JavaScript code?

var hero = {
  _name: "John Doe",
  getSecretIdentity: function () {
    return this._name;
  },
};
var stoleSecretIdentity = hero.getSecretIdentity;

console.log(stoleSecretIdentity()); // Output: undefined
console.log(hero.getSecretIdentity()); // Output: John Doe

Q. Predict the output of the following JavaScript code?

var length = 10;
function fn() {
  console.log(this.length);
}

var obj = {
  length: 5,
  method: function (fn) {
    fn();
    arguments[0]();
  },
};

obj.method(fn, 1);
//Output: 10, 2

Q. Predict the output of the following JavaScript code?

(function () {
  try {
    throw new Error();
  } catch (x) {
    var x = 1,
      y = 2;
    console.log(x);
  }
  console.log(x);
  console.log(y);
})();
//Output:  1, undefined, 2

Q. Predict the output of the following JavaScript code?

var x = 21;
var girl = function () {
  console.log(x); // Output: undefined
  var x = 20;
};
girl();

Q. Predict the output of the following JavaScript code?

console.log(1 < 2 < 3); // Output: true
console.log(3 > 2 > 1); // Output: false

Q. Predict the output of the following JavaScript code?

console.log(typeof typeof 1); // Output: string

Q. Predict the output of the following JavaScript code?

var b = 1;
function outer() {
  var b = 2;
  function inner() {
    b++;
    var b = 3;
    console.log(b); //3
  }
  inner();
}
outer();

Q. Hoisting example in javascript?

x = 10;
console.log(x);
var x; // Output: 10

Q. Predict the output of the following JavaScript code?

const arr = [1, 2];
arr.push(3); // Output: 1, 2, 3

Q. Predict the output of the following JavaScript code?

var o = new F();
o.constructor === F;

Q. Predict the output of the following JavaScript code?

let sum = (a, b) => {
  a + b;
};
console.log(sum(10, 20)); // Output: undefined; return keyword is missing

Q. Predict the output of the following JavaScript code?

var arr = ["javascript", "typescript", "es6"];
var searchValue = (value) => {
  return arr.filter((item) => {
    return item.indexOf(value) > -1;
  });
};
console.log(searchValue("script"));

Q. Write the program using fatarrow function?

var a = [1, 2, 3, 4];
function sumUsingFunction(acc, value) {
  return acc + value;
}
var sumOfArrayUsingFunc = a.reduce(sumUsingFunc);

Q. Write a program that prints the numbers from 1 to 15. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”?

for (var i = 1; i <= 15; i++) {
  if (i % 15 == 0) console.log("FizzBuzz");
  else if (i % 3 == 0) console.log("Fizz");
  else if (i % 5 == 0) console.log("Buzz");
  else console.log(i);
}

Output:

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

Solution – 02

for (var i = 1; i <= 15; i++) {
  var f = i % 3 == 0,
    b = i % 5 == 0;
  console.log(f ? (b ? "FizzBuzz" : "Fizz") : b ? "Buzz" : i);
}

Q. What will be the output of the following code?

var output = (function (x) {
  delete x;
  return x;
})(0);

console.log(output);

The code above will output 0 as output. delete operator is used to delete a property from an object. Here x is not an object it’s local variabledelete operator doesn’t affect local variables.

Q. What will be the output of the following code?

var x = 1;
var output = (function () {
  delete x;
  return x;
})();

console.log(output);

The code above will output 1 as output. delete operator is used to delete a property from an object. Here x is not an object it’s global variable of type number.

Q. What will be the output of the following code?

var x = { foo: 1 };
var output = (function () {
  delete x.foo;
  return x.foo;
})();

console.log(output);

The code above will output undefined as output. delete operator is used to delete a property from an object. Here x is an object which has foo as a property and from a self-invoking function, we are deleting the foo property of object x and after deletion, we are trying to reference deleted property foo which result undefined.

Q. What will be the output of the following code?

var Employee = {
  company: "xyz",
};
var emp1 = Object.create(Employee);
delete emp1.company;
console.log(emp1.company);

The code above will output xyz as output. Here emp1 object got company as prototype property. delete operator doesn’t delete prototype property.

emp1 object doesn’t have company as its own property. you can test it console.log(emp1.hasOwnProperty('company')); //output : false However, we can delete company property directly from Employee object using delete Employee.company or we can also delete from emp1 object using __proto__ property delete emp1.__proto__.company.

Q. What will be the output of the following code?

var trees = ["xyz", "xxxx", "test", "ryan", "apple"];
delete trees[3];
console.log(trees.length);

The code above will output 5 as output. When we used delete operator for deleting an array element then, the array length is not affected by this. This holds even if you deleted all elements of an array using delete operator.

So when delete operator removes an array element that deleted element is no longer present in the array. In place of value at deleted index undefined x 1 in chrome and undefined is placed at the index. If you do console.log(trees) output ["xyz", "xxxx", "test", undefined × 1, "apple"] in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"].

Q. What will be the output of the following code?

var bar = true;
console.log(bar + 0);
console.log(bar + "xyz");
console.log(bar + true);
console.log(bar + false);

The code above will output 1, "truexyz", 2, 1 as output. Here’s a general guideline for the plus operator:

  • Number + Number -> Addition
  • Boolean + Number -> Addition
  • Boolean + Boolean -> Addition
  • Number + String -> Concatenation
  • String + Boolean -> Concatenation
  • String + String -> Concatenation

Q. What will be the output of the following code?

var z = 1,
  y = (z = typeof y);
console.log(y);

The code above will print string "undefined" as output. According to associativity rule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator is Right to Left so first typeof y will evaluate first which is string "undefined" and assigned to z and then y would be assigned the value of z. The overall sequence will look like that:

var z;
z = 1;
var y;
z = typeof y;
y = z;

Q. What will be the output of the following code?

// NFE (Named Function Expression)
var foo = function bar() {
  return 12;
};
typeof bar();

The output will be Reference Error. To fix the bug we can try to rewrite the code a little bit:

Sample 1

var bar = function () {
  return 12;
};
typeof bar();

or

Sample 2

function bar() {
  return 12;
}
typeof bar();

The function definition can have only one reference variable as a function name, In sample 1 bar is reference variable which is pointing to anonymous function and in sample 2 we have function statement and bar is the function name.

var foo = function bar() {
  // foo is visible here
  // bar is visible here
  console.log(typeof bar()); // Works here :)
};
// foo is visible here
// bar is undefined here

Q. What is the output of the following?

bar();
(function abc() {
  console.log("something");
})();
function bar() {
  console.log("bar got called");
}

The output will be :

bar got called
something

Since the function is called first and defined during parse time the JS engine will try to find any possible parse time definitions and start the execution loop which will mean function is called first even if the definition is post another function.

Q. What will be the output of the following code?

var salary = "1000$";

(function () {
  console.log("Original salary was " + salary);

  var salary = "5000$";

  console.log("My New Salary " + salary);
})();

The code above will output: undefined, 5000$ because of hoisting. In the code presented above, you might be expecting salary to retain it values from outer scope until the point that salary was re-declared in the inner scope. But due to hoisting salary value was undefined instead. To understand it better have a look of the following code, here salary variable is hoisted and declared at the top in function scope. When we print its value using console.log the result is undefined. Afterwards the variable is redeclared and the new value "5000$" is assigned to it.

var salary = "1000$";

(function () {
  var salary = undefined;
  console.log("Original salary was " + salary);

  salary = "5000$";

  console.log("My New Salary " + salary);
})();

Q. What would be the output of the following code?

function User(name) {
  this.name = name || "JsGeeks";
}

var person = (new User("xyz")["location"] = "USA");
console.log(person);

The output of above code would be "USA". Here new User("xyz") creates a brand new object and created property location on that and USA has been assigned to object property location and that has been referenced by the person.

Let say new User("xyz") created a object called foo. The value "USA" will be assigned to foo["location"], but according to ECMAScript Specification , pt 12.14.4 the assignment will itself return the rightmost value: in our case it’s "USA". Then it will be assigned to person.

To better understand What is going on here, try to execute this code in console, line by line:

function User(name) {
  this.name = name || "JsGeeks";
}

var person;
var foo = new User("xyz");
foo["location"] = "USA";
// the console will show you that the result of this is "USA"

Q. What would be the output of following code?

var strA = "hi there";
var strB = strA;
strB = "bye there!";
console.log(strA);

The output will 'hi there' because we’re dealing with strings here. Strings are passed by value, that is, copied.

Q. What would be the output of following code?

var objA = { prop1: 42 };
var objB = objA;
objB.prop1 = 90;
console.log(objA);

The output will {prop1: 90} because we’re dealing with objects here. Objects are passed by reference, that is, objA and objB point to the same object in memory.

Q. What would be the output of following code?

var objA = { prop1: 42 };
var objB = objA;
objB = {};
console.log(objA);

The output will {prop1: 42}.

When we assign objA to objB, the objB variable will point to the same object as the objB variable.

However, when we reassign objB to an empty object, we simply change where objB variable references to. This doesn’t affect where objA variable references to.

Q. What would be the output of following code?

var arrA = [0, 1, 2, 3, 4, 5];
var arrB = arrA;
arrB[0] = 42;
console.log(arrA);

The output will be [42,1,2,3,4,5].

Arrays are object in JavaScript and they are passed and assigned by reference. This is why both arrA and arrB point to the same array [0,1,2,3,4,5]. That’s why changing the first element of the arrB will also modify arrA: it’s the same array in the memory.

Q. What would be the output of following code?

var arrA = [0, 1, 2, 3, 4, 5];
var arrB = arrA.slice();
arrB[0] = 42;
console.log(arrA);

The output will be [0,1,2,3,4,5].

The slice function copies all the elements of the array returning the new array. That’s why arrA and arrB reference two completely different arrays.

Q. What would be the output of following code?

var arrA = [
  { prop1: "value of array A!!" },
  { someProp: "also value of array A!" },
  3,
  4,
  5,
];
var arrB = arrA;
arrB[0].prop1 = 42;
console.log(arrA);

The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5].

Arrays are object in JS, so both varaibles arrA and arrB point to the same array. Changing arrB[0] is the same as changing arrA[0]

Q. What would be the output of following code?

var arrA = [
  { prop1: "value of array A!!" },
  { someProp: "also value of array A!" },
  3,
  4,
  5,
];
var arrB = arrA.slice();
arrB[0].prop1 = 42;
arrB[3] = 20;
console.log(arrA);

The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5].

The slice function copies all the elements of the array returning the new array. However, it doesn’t do deep copying. Instead it does shallow copying. You can imagine slice implemented like this:

function slice(arr) {
  var result = [];
  for (i = 0; i < arr.length; i++) {
    result.push(arr[i]);
  }
  return result;
}

Look at the line with result.push(arr[i]). If arr[i] happens to be a number or string, it will be passed by value, in other words, copied. If arr[i] is an object, it will be passed by reference.

In case of our array arr[0] is an object {prop1: "value of array A!!"}. Only the reference to this object will be copied. This effectively means that arrays arrA and arrB share first two elements.

This is why changing the property of arrB[0] in arrB will also change the arrA[0].

Q. console.log(employeeId);

  1. Some Value
  2. Undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 4) ReferenceError: employeeId is not defined

Q. What would be the output of following code?

console.log(employeeId);
var employeeId = "19000";
  1. Some Value
  2. undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

var employeeId = "1234abe";
(function () {
  console.log(employeeId);
  var employeeId = "122345";
})();
  1. ‘122345’
  2. undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

var employeeId = "1234abe";
(function () {
  console.log(employeeId);
  var employeeId = "122345";
  (function () {
    var employeeId = "abc1234";
  })();
})();
  1. ‘122345’
  2. undefined
  3. ‘1234abe’
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

(function () {
  console.log(typeof displayFunc);
  var displayFunc = function () {
    console.log("Hi I am inside displayFunc");
  };
})();
  1. undefined
  2. function
  3. ‘Hi I am inside displayFunc’
  4. ReferenceError: displayFunc is not defined

Answer: 1) undefined

Q. What would be the output of following code?

var employeeId = "abc123";
function foo() {
  employeeId = "123bcd";
  return;
}
foo();
console.log(employeeId);
  1. undefined
  2. ‘123bcd’
  3. ‘abc123’
  4. ReferenceError: employeeId is not defined

Answer: 2) ‘123bcd’

Q. What would be the output of following code?

var employeeId = "abc123";

function foo() {
  employeeId = "123bcd";
  return;

  function employeeId() {}
}
foo();
console.log(employeeId);
  1. undefined
  2. ‘123bcd’
  3. ‘abc123’
  4. ReferenceError: employeeId is not defined

Answer: 3) ‘abc123’

Q. What would be the output of following code?

var employeeId = "abc123";

function foo() {
  employeeId();
  return;

  function employeeId() {
    console.log(typeof employeeId);
  }
}
foo();
  1. undefined
  2. function
  3. string
  4. ReferenceError: employeeId is not defined

Answer: 2) ‘function’

Q. What would be the output of following code?

function foo() {
  employeeId();
  var product = "Car";
  return;

  function employeeId() {
    console.log(product);
  }
}
foo();
  1. undefined
  2. Type Error
  3. ‘Car’
  4. ReferenceError: product is not defined

Answer: 1) undefined

Q. What would be the output of following code?

(function foo() {
  bar();

  function bar() {
    abc();
    console.log(typeof abc);
  }

  function abc() {
    console.log(typeof bar);
  }
})();
  1. undefined undefined
  2. Type Error
  3. function function
  4. ReferenceError: bar is not defined

Answer: 3) function function

Q. What would be the output of following code?

(function () {
  "use strict";

  var person = {
    name: "John",
  };
  person.salary = "10000$";
  person["country"] = "USA";

  Object.defineProperty(person, "phoneNo", {
    value: "8888888888",
    enumerable: true,
  });

  console.log(Object.keys(person));
})();
  1. Type Error
  2. undefined
  3. [“name”, “salary”, “country”, “phoneNo”]
  4. [“name”, “salary”, “country”]

Answer: 3) [“name”, “salary”, “country”, “phoneNo”]

Q. What would be the output of following code?

(function () {
  "use strict";

  var person = {
    name: "John",
  };
  person.salary = "10000$";
  person["country"] = "USA";

  Object.defineProperty(person, "phoneNo", {
    value: "8888888888",
    enumerable: false,
  });

  console.log(Object.keys(person));
})();
  1. Type Error
  2. undefined
  3. [“name”, “salary”, “country”, “phoneNo”]
  4. [“name”, “salary”, “country”]

Answer: 4) [“name”, “salary”, “country”]

Q. What would be the output of following code?

(function () {
  var objA = {
    foo: "foo",
    bar: "bar",
  };
  var objB = {
    foo: "foo",
    bar: "bar",
  };
  console.log(objA == objB);
  console.log(objA === objB);
})();
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code?

(function () {
  var objA = new Object({ foo: "foo" });
  var objB = new Object({ foo: "foo" });
  console.log(objA == objB);
  console.log(objA === objB);
})();
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = Object.create({
    foo: "foo",
  });
  console.log(objA == objB);
  console.log(objA === objB);
})();
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = Object.create(objA);
  console.log(objA == objB);
  console.log(objA === objB);
})();
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = Object.create(objA);
  console.log(objA.toString() == objB.toString());
  console.log(objA.toString() === objB.toString());
})();
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 4) true true

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = objA;
  console.log(objA == objB);
  console.log(objA === objB);
  console.log(objA.toString() == objB.toString());
  console.log(objA.toString() === objB.toString());
})();
  1. true true true false
  2. true false true true
  3. true true true true
  4. true true false false

Answer: 3) true true true true

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = objA;
  objB.foo = "bar";
  console.log(objA.foo);
  console.log(objB.foo);
})();
  1. foo bar
  2. bar bar
  3. foo foo
  4. bar foo

Answer: 2) bar bar

Q. What would be the output of following code?

(function () {
  var objA = Object.create({
    foo: "foo",
  });
  var objB = objA;
  objB.foo = "bar";

  delete objA.foo;
  console.log(objA.foo);
  console.log(objB.foo);
})();
  1. foo bar
  2. bar bar
  3. foo foo
  4. bar foo

Answer: 3) foo foo

Q. What would be the output of following code?

(function () {
  var objA = {
    foo: "foo",
  };
  var objB = objA;
  objB.foo = "bar";

  delete objA.foo;
  console.log(objA.foo);
  console.log(objB.foo);
})();
  1. foo bar
  2. undefined undefined
  3. foo foo
  4. undefined bar

Answer: 2) undefined undefined

Q. What would be the output of following code?

(function () {
  var array = new Array("100");
  console.log(array);
  console.log(array.length);
})();
  1. undefined undefined
  2. [undefined × 100] 100
  3. [“100”] 1
  4. ReferenceError: array is not defined

Answer: 3) [“100”] 1

Q. What would be the output of following code?

(function () {
  var array1 = [];
  var array2 = new Array(100);
  var array3 = new Array(["1", 2, "3", 4, 5.6]);
  console.log(array1);
  console.log(array2);
  console.log(array3);
  console.log(array3.length);
})();
  1. [] [] [Array[5]] 1
  2. [] [undefined × 100] Array[5] 1
  3. [] [] [‘1′,2,’3’,4,5.6] 5
  4. [] [] [Array[5]] 5

Answer: 1) [] [] [Array[5]] 1

Q. What would be the output of following code?

(function () {
  var array = new Array("a", "b", "c", "d", "e");
  array[10] = "f";
  delete array[10];
  console.log(array.length);
})();
  1. 11
  2. 5
  3. 6
  4. undefined

Answer: 1) 11

Q. What would be the output of following code?

(function () {
  var animal = ["cow", "horse"];
  animal.push("cat");
  animal.push("dog", "rat", "goat");
  console.log(animal.length);
})();
  1. 4
  2. 5
  3. 6
  4. undefined

Answer: 3) 6

Q. What would be the output of following code?

(function () {
  var animal = ["cow", "horse"];
  animal.push("cat");
  animal.unshift("dog", "rat", "goat");
  console.log(animal);
})();
  1. [ ‘dog’, ‘rat’, ‘goat’, ‘cow’, ‘horse’, ‘cat’ ]
  2. [ ‘cow’, ‘horse’, ‘cat’, ‘dog’, ‘rat’, ‘goat’ ]
  3. Type Error
  4. undefined

Answer: 1) [ ‘dog’, ‘rat’, ‘goat’, ‘cow’, ‘horse’, ‘cat’ ]

Q. What would be the output of following code?

(function () {
  var array = [1, 2, 3, 4, 5];
  console.log(array.indexOf(2));
  console.log([{ name: "John" }, { name: "John" }].indexOf({ name: "John" }));
  console.log([[1], [2], [3], [4]].indexOf([3]));
  console.log("abcdefgh".indexOf("e"));
})();
  1. 1 -1 -1 4
  2. 1 0 -1 4
  3. 1 -1 -1 -1
  4. 1 undefined -1 4

Answer: 1) 1 -1 -1 4

Q. What would be the output of following code?

(function () {
  var array = [1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 6];
  console.log(array.indexOf(2));
  console.log(array.indexOf(2, 3));
  console.log(array.indexOf(2, 10));
})();
  1. 1 -1 -1
  2. 1 6 -1
  3. 1 1 -1
  4. 1 undefined undefined

Answer: 2) 1 6 -1

Q. What would be the output of following code?

(function () {
  var numbers = [2, 3, 4, 8, 9, 11, 13, 12, 16];
  var even = numbers.filter(function (element, index) {
    return element % 2 === 0;
  });
  console.log(even);

  var containsDivisibleby3 = numbers.some(function (element, index) {
    return element % 3 === 0;
  });

  console.log(containsDivisibleby3);
})();
  1. [ 2, 4, 8, 12, 16 ] [ 0, 3, 0, 0, 9, 0, 12]
  2. [ 2, 4, 8, 12, 16 ] [ 3, 9, 12]
  3. [ 2, 4, 8, 12, 16 ] true
  4. [ 2, 4, 8, 12, 16 ] false

Answer: 3) [ 2, 4, 8, 12, 16 ] true

Q. What would be the output of following code?

(function () {
  var containers = [2, 0, false, "", "12", true];
  var containers = containers.filter(Boolean);
  console.log(containers);
  var containers = containers.filter(Number);
  console.log(containers);
  var containers = containers.filter(String);
  console.log(containers);
  var containers = containers.filter(Object);
  console.log(containers);
})();
  1. [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ]
  2. [false, true] [ 2 ] [’12’] [ ]
  3. [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true]
  4. [ 2, ’12’, true ] [ 2, ’12’, true, false ] [ 2, ’12’, true,false ] [ 2, ’12’, true,false]

Answer: 1) [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ]

Q. What would be the output of following code?

(function () {
  var list = ["foo", "bar", "john", "ritz"];
  console.log(list.slice(1));
  console.log(list.slice(1, 3));
  console.log(list.slice());
  console.log(list.slice(2, 2));
  console.log(list);
})();
  1. [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
  2. [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’,’ritz ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
  3. [ ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
  4. [ ‘foo’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]

Answer: 1) [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]

Q. What would be the output of following code?

(function () {
  var list = ["foo", "bar", "john"];
  console.log(list.splice(1));
  console.log(list.splice(1, 2));
  console.log(list);
})();
  1. [ ‘bar’, ‘john’ ] [] [ ‘foo’ ]
  2. [ ‘bar’, ‘john’ ] [] [ ‘bar’, ‘john’ ]
  3. [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ]
  4. [ ‘bar’, ‘john’ ] [] []

Answer: 1. [ ‘bar’, ‘john’ ] [] [ ‘foo’ ]

Q. What would be the output of following code?

(function () {
  var arrayNumb = [2, 8, 15, 16, 23, 42];
  arrayNumb.sort();
  console.log(arrayNumb);
})();
  1. [2, 8, 15, 16, 23, 42]
  2. [42, 23, 26, 15, 8, 2]
  3. [ 15, 16, 2, 23, 42, 8 ]
  4. [ 2, 8, 15, 16, 23, 42 ]

Answer: 3. [ 15, 16, 2, 23, 42, 8 ]

Q. What would be the output of following code?

function funcA() {
  console.log("funcA ", this);
  (function innerFuncA1() {
    console.log("innerFunc1", this);
    (function innerFunA11() {
      console.log("innerFunA11", this);
    })();
  })();
}

console.log(funcA());
  1. funcA Window {…} innerFunc1 Window {…} innerFunA11 Window {…}
  2. undefined
  3. Type Error
  4. ReferenceError: this is not defined

Answer: 1)

Q. What would be the output of following code?

var obj = {
  message: "Hello",
  innerMessage: !(function () {
    console.log(this.message);
  })(),
};

console.log(obj.innerMessage);
  1. ReferenceError: this.message is not defined
  2. undefined
  3. Type Error
  4. undefined true

Answer: 4) undefined true

Q. What would be the output of following code?

var obj = {
  message: "Hello",
  innerMessage: function () {
    return this.message;
  },
};

console.log(obj.innerMessage());
  1. Hello
  2. undefined
  3. Type Error
  4. ReferenceError: this.message is not defined

Answer: 1) Hello

Q. What would be the output of following code?

var obj = {
  message: "Hello",
  innerMessage: function () {
    (function () {
      console.log(this.message);
    })();
  },
};
console.log(obj.innerMessage());
  1. Type Error
  2. Hello
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 3) undefined

Q. What would be the output of following code?

var obj = {
  message: "Hello",
  innerMessage: function () {
    var self = this;
    (function () {
      console.log(self.message);
    })();
  },
};
console.log(obj.innerMessage());
  1. Type Error
  2. ‘Hello’
  3. undefined
  4. ReferenceError: self.message is not defined

Answer: 2) ‘Hello’

Q. What would be the output of following code?

function myFunc() {
  console.log(this.message);
}
myFunc.message = "Hi John";

console.log(myFunc());
  1. Type Error
  2. ‘Hi John’
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 3) undefined

Q. What would be the output of following code?

function myFunc() {
  console.log(myFunc.message);
}
myFunc.message = "Hi John";

console.log(myFunc());
  1. Type Error
  2. ‘Hi John’
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 2) ‘Hi John’

Q. What would be the output of following code?

function myFunc() {
  myFunc.message = "Hi John";
  console.log(myFunc.message);
}
console.log(myFunc());
  1. Type Error
  2. ‘Hi John’
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 2) ‘Hi John’

Q. What would be the output of following code?

function myFunc(param1, param2) {
  console.log(myFunc.length);
}
console.log(myFunc());
console.log(myFunc("a", "b"));
console.log(myFunc("a", "b", "c", "d"));
  1. 2 2 2
  2. 0 2 4
  3. undefined
  4. ReferenceError

Answer: a) 2 2 2

Q. What would be the output of following code?

function myFunc() {
  console.log(arguments.length);
}
console.log(myFunc());
console.log(myFunc("a", "b"));
console.log(myFunc("a", "b", "c", "d"));
  1. 2 2 2
  2. 0 2 4
  3. undefined
  4. ReferenceError

Answer: 2) 0 2 4

Q. What would be the output of following code?

function Person(name, age) {
  this.name = name || "John";
  this.age = age || 24;
  this.displayName = function () {
    console.log(this.name);
  };
}

Person.name = "John";
Person.displayName = function () {
  console.log(this.name);
};

var person1 = new Person("John");
person1.displayName();
Person.displayName();
  1. John Person
  2. John John
  3. John undefined
  4. John John

Answer: 1) John Person

Q. What would be the output of following code?

function passWordMngr() {
  var password = "12345678";
  this.userName = "John";
  return {
    pwd: password,
  };
}
// Block End
var userInfo = passWordMngr();
console.log(userInfo.pwd);
console.log(userInfo.userName);
  1. 12345678 Window
  2. 12345678 John
  3. 12345678 undefined
  4. undefined undefined

Answer: 3) 12345678 undefined

Q. What would be the output of following code?

var employeeId = "aq123";
function Employee() {
  this.employeeId = "bq1uy";
}
console.log(Employee.employeeId);
  1. Reference Error
  2. aq123
  3. bq1uy
  4. undefined

Answer: 4) undefined

Q. What would be the output of following code?

var employeeId = "aq123";

function Employee() {
  this.employeeId = "bq1uy";
}
console.log(new Employee().employeeId);
Employee.prototype.employeeId = "kj182";
Employee.prototype.JobId = "1BJKSJ";
console.log(new Employee().JobId);
console.log(new Employee().employeeId);
  1. bq1uy 1BJKSJ bq1uy undefined
  2. bq1uy 1BJKSJ bq1uy
  3. bq1uy 1BJKSJ kj182
  4. undefined 1BJKSJ kj182

Answer: 2) bq1uy 1BJKSJ bq1uy

Q. What would be the output of following code?

var employeeId = "aq123";
(function Employee() {
  try {
    throw "foo123";
  } catch (employeeId) {
    console.log(employeeId);
  }
  console.log(employeeId);
})();
  1. foo123 aq123
  2. foo123 foo123
  3. aq123 aq123
  4. foo123 undefined

Answer: 1) foo123 aq123

Q. What would be the output of following code?

(function () {
  var greet = "Hello World";
  var toGreet = [].filter.call(greet, function (element, index) {
    return index > 5;
  });
  console.log(toGreet);
})();
  1. Hello World
  2. undefined
  3. World
  4. [ ‘W’, ‘o’, ‘r’, ‘l’, ‘d’ ]

Answer: 4) [ ‘W’, ‘o’, ‘r’, ‘l’, ‘d’ ]

Q. What would be the output of following code?

(function () {
  var fooAccount = {
    name: "John",
    amount: 4000,
    deductAmount: function (amount) {
      this.amount -= amount;
      return "Total amount left in account: " + this.amount;
    },
  };
  var barAccount = {
    name: "John",
    amount: 6000,
  };
  var withdrawAmountBy = function (totalAmount) {
    return fooAccount.deductAmount.bind(barAccount, totalAmount);
  };
  console.log(withdrawAmountBy(400)());
  console.log(withdrawAmountBy(300)());
})();
  1. Total amount left in account: 5600 Total amount left in account: 5300
  2. undefined undefined
  3. Total amount left in account: 3600 Total amount left in account: 3300
  4. Total amount left in account: 5600 Total amount left in account: 5600

Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300

Q. What would be the output of following code?

(function () {
  var fooAccount = {
    name: "John",
    amount: 4000,
    deductAmount: function (amount) {
      this.amount -= amount;
      return this.amount;
    },
  };
  var barAccount = {
    name: "John",
    amount: 6000,
  };
  var withdrawAmountBy = function (totalAmount) {
    return fooAccount.deductAmount.apply(barAccount, [totalAmount]);
  };
  console.log(withdrawAmountBy(400));
  console.log(withdrawAmountBy(300));
  console.log(withdrawAmountBy(200));
})();
  1. 5600 5300 5100
  2. 3600 3300 3100
  3. 5600 3300 5100
  4. undefined undefined undefined

Answer: 1) 5600 5300 5100

Q. What would be the output of following code?

(function () {
  var fooAccount = {
    name: "John",
    amount: 6000,
    deductAmount: function (amount) {
      this.amount -= amount;
      return this.amount;
    },
  };
  var barAccount = {
    name: "John",
    amount: 4000,
  };
  var withdrawAmountBy = function (totalAmount) {
    return fooAccount.deductAmount.call(barAccount, totalAmount);
  };
  console.log(withdrawAmountBy(400));
  console.log(withdrawAmountBy(300));
  console.log(withdrawAmountBy(200));
})();
  1. 5600 5300 5100
  2. 3600 3300 3100
  3. 5600 3300 5100
  4. undefined undefined undefined

Answer: 2) 3600 3300 3100

Q. What would be the output of following code?

(function greetNewCustomer() {
  console.log("Hello " + this.name);
}.bind({
  name: "John",
})());
  1. Hello John
  2. Reference Error
  3. Window
  4. undefined

Answer: 1) Hello John

Q. What would be the output of following code?

function getDataFromServer(apiUrl) {
  var name = "John";
  return {
    then: function (fn) {
      fn(name);
    },
  };
}

getDataFromServer("www.google.com").then(function (name) {
  console.log(name);
});
  1. John
  2. undefined
  3. Reference Error
  4. fn is not defined

Answer: 1) John

Q. What would be the output of following code?

(function () {
  var arrayNumb = [2, 8, 15, 16, 23, 42];
  Array.prototype.sort = function (a, b) {
    return a - b;
  };
  arrayNumb.sort();
  console.log(arrayNumb);
})();

(function () {
  var numberArray = [2, 8, 15, 16, 23, 42];
  numberArray.sort(function (a, b) {
    if (a == b) {
      return 0;
    } else {
      return a < b ? -1 : 1;
    }
  });
  console.log(numberArray);
})();

(function () {
  var numberArray = [2, 8, 15, 16, 23, 42];
  numberArray.sort(function (a, b) {
    return a - b;
  });
  console.log(numberArray);
})();
  1. [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
  2. undefined undefined undefined
  3. [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2]
  4. Reference Error

Answer: 1) [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]

Q. What would be the output of following code?

(function () {
  function sayHello() {
    var name = "Hi John";
    return;
    {
      fullName: name;
    }
  }
  console.log(sayHello().fullName);
})();
  1. Hi John
  2. undefined
  3. Reference Error
  4. Uncaught TypeError: Cannot read property ‘fullName’ of undefined

Answer: 4) Uncaught TypeError: Cannot read property ‘fullName’ of undefined

Q. What would be the output of following code?

function getNumber() {
  return 2, 4, 5;
}

var numb = getNumber();
console.log(numb);
  1. 5
  2. undefined
  3. 2
  4. (2,4,5)

Answer: 1) 5

Q. What would be the output of following code?

function getNumber() {
  return;
}

var numb = getNumber();
console.log(numb);
  1. null
  2. undefined
  3. “”
  4. 0

Answer: 2) undefined

Q. What would be the output of following code?

function mul(x) {
  return function (y) {
    return [
      x * y,
      function (z) {
        return x * y + z;
      },
    ];
  };
}

console.log(mul(2)(3)[0]);
console.log(mul(2)(3)[1](4));
  1. 6, 10
  2. undefined undefined
  3. Reference Error
  4. 10, 6

Answer: 1) 6, 10

Q. What would be the output of following code?

function mul(x) {
  return function (y) {
    return {
      result: x * y,
      sum: function (z) {
        return x * y + z;
      },
    };
  };
}
console.log(mul(2)(3).result);
console.log(mul(2)(3).sum(4));
  1. 6, 10
  2. undefined undefined
  3. Reference Error
  4. 10, 6

Answer: 1) 6, 10

Q. What would be the output of following code?

function mul(x) {
  return function (y) {
    return function (z) {
      return function (w) {
        return function (p) {
          return x * y * z * w * p;
        };
      };
    };
  };
}
console.log(mul(2)(3)(4)(5)(6));
  1. 720
  2. undefined
  3. Reference Error
  4. Type Error

Answer: 1) 720

Q. What is the value of foo?

var foo = 10 + "20";

Answer: '1020', because of type coercion from Number to String

Q. How would you make this work?

add(2, 5); // 7
add(2)(5); // 7

Answer: A general solution for any number of parameters

"use strict";

let sum = (arr) => arr.reduce((a, b) => a + b);
let addGenerator = (numArgs, prevArgs) => {
  return function () {
    let totalArgs = prevArgs.concat(Array.from(arguments));
    if (totalArgs.length === numArgs) {
      return sum(totalArgs);
    }
    return addGenerator(numArgs, totalArgs);
  };
};

let add = addGenerator(2, []);

add(2, 5); // 7
add(2)(5); // 7
add()(2, 5); // 7
add()(2)(5); // 7
add()()(2)(5); // 7

Q. What value is returned from the following statement?

"i'm a lasagna hog".split("").reverse().join("");

Answer: It’s actually a reverse method for a string – 'goh angasal a m\'i'

Q. What is the value of window.foo?

window.foo || (window.foo = "bar");

Answer: Always 'bar'

Q. What is the outcome of the two alerts below?

var foo = "Hello";
(function () {
  var bar = " World";
  alert(foo + bar);
})();
alert(foo + bar);

Answer:

  • First: Hello World
  • Second: Throws an exception, ReferenceError: bar is not defined

Q. What is the value of foo.length?

var foo = [];
foo.push(1);
foo.push(2);

Answer: .push is mutable – 2

Q. What is the value of foo.x?

var foo = { n: 1 };
var bar = foo;
foo.x = foo = { n: 2 };

Answer: undefined. Rather, bar.x is {n: 2}.

foo.x = foo = {n: 2} is the same as foo.x = (foo = {n: 2}). It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x is referenced, it refers to an original object, {n: 1}. So, when the result of the right term, {n: 2}, is evaluated, it will assigned to the original object, which is at the moment referenced by bar.

Q. What does the following code print?

console.log("one");
setTimeout(function () {
  console.log("two");
}, 0);
console.log("three");

Answer: onethree and two. It’s because console.log('two'); will be invoked in the next event loop.

Q. What would be the result of 1+2+’3′?

The output is going to be 33. Since 1 and 2 are numeric values, the result of first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.

Q. Write a script that returns the number of occurrences of character given a string as input?

function countCharacters(str) {
  return str
    .replace(/ /g, "")
    .toLowerCase()
    .split("")
    .reduce((arr, character) => {
      if (character in arr) {
        arr[character]++;
      } else {
        arr[character] = 1;
      }
      return arr;
    }, {});
}
console.log(countCharacters("the brown fox jumps over the lazy dog"));

Q. What is the value of foo?

var foo = 10 + "20";

Answer: '1020', because of type coercion from Number to String

Q. How would you make this work?

add(2, 5); // 7
add(2)(5); // 7

Answer: A general solution for any number of parameters

"use strict";

let sum = (arr) => arr.reduce((a, b) => a + b);
let addGenerator = (numArgs, prevArgs) => {
  return function () {
    let totalArgs = prevArgs.concat(Array.from(arguments));
    if (totalArgs.length === numArgs) {
      return sum(totalArgs);
    }
    return addGenerator(numArgs, totalArgs);
  };
};

let add = addGenerator(2, []);

add(2, 5); // 7
add(2)(5); // 7
add()(2, 5); // 7
add()(2)(5); // 7
add()()(2)(5); // 7

Q. What value is returned from the following statement?

"i'm a lasagna hog".split("").reverse().join("");

Answer: It’s actually a reverse method for a string – 'goh angasal a m\'i'

Q. What is the value of window.foo?

window.foo || (window.foo = "bar");

Answer: Always 'bar'

Q. What is the outcome of the two alerts below?

var foo = "Hello";
(function () {
  var bar = " World";
  alert(foo + bar);
})();
alert(foo + bar);

Answer:

  • First: Hello World
  • Second: Throws an exception, ReferenceError: bar is not defined

Q. What is the value of foo.length?

var foo = [];
foo.push(1);
foo.push(2);

Answer: .push is mutable – 2

Q. What is the value of foo.x?

var foo = { n: 1 };
var bar = foo;
foo.x = foo = { n: 2 };

Answer: undefined. Rather, bar.x is {n: 2}.

foo.x = foo = {n: 2} is the same as foo.x = (foo = {n: 2}). It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x is referenced, it refers to an original object, {n: 1}. So, when the result of the right term, {n: 2}, is evaluated, it will assigned to the original object, which is at the moment referenced by bar.

Q. What does the following code print?

console.log("one");
setTimeout(function () {
  console.log("two");
}, 0);
console.log("three");

Answer: onethree and two. It’s because console.log('two'); will be invoked in the next event loop.

Q. For which value of x the results of the following statements are not the same?

//  if( x <= 100 ) {...}
if( !(x > 100) ) {...}

Answer: NaN <= 100 is false and NaN > 100 is also false, so if the value of x is NaN, the statements are not the same.

The same holds true for any value of x that being converted to Number, returns NaN, e.g.: undefined[1,2,5]{a:22}, etc.

Q. What is g value?

f = g = 0;
(function () {
  try {
    f =
      function () {
        return f();
      } && f();
  } catch (e) {
    return g++ && f();
  } finally {
    return ++g;
  }
  function f() {
    g += 5;
    return 0;
  }
})();

Q. What will be the output?

function b(b) {
  return this.b && b(b);
}
b(b.bind(b));

Q. What will be the output?

c = (c) => {
  return this.c && c(c);
};
c(c.bind(c));

Q. Predict the output of the following JavaScript code?

var g = 0;
g = 1 && g++;
console.log(g);

Q. Predict the output of the following JavaScript code?

!function(){}()
function(){}()
true && function(){}()
(function(){})()
function(){}
!function(){}

Q. ***What will expression return?

var a = (b = true),
  c = (a) => a;
(function a(a = (c(b).a = c = () => a)) {
  return a();
})();

Q. Predict the output of the following JavaScript code?

var a = true;
(a = function () {
  return a;
})();

Q. What will be the output?

var v = 0;
try {
  throw (v = (function (c) {
    throw (v = function (a) {
      return v;
    });
  })());
} catch (e) {
  console.log(e()());
}

Q. What will the following code output?

const arr = [10, 12, 15, 21];
for (var i = 0; i < arr.length; i++) {
  setTimeout(function () {
    console.log("Index: " + i + ", element: " + arr[i]);
  }, 3000);
}

Q. What will be the output of the following code?

var output = (function (x) {
  delete x;
  return x;
})(0);

console.log(output);

Q. What will be the output of the following code?

var Employee = {
  company: "xyz",
};
var emp1 = Object.create(Employee);
delete emp1.company;
console.log(emp1.company);

Q. ***Make this work:

duplicate([1, 2, 3, 4, 5]); // [1,2,3,4,5,1,2,3,4,5]
function duplicate(arr) {
  return arr.concat(arr);
}

duplicate([1, 2, 3, 4, 5]); // [1,2,3,4,5,1,2,3,4,5]

Q. Fix the bug using ES5 only?

var arr = [10, 32, 65, 2];
for (var i = 0; i < arr.length; i++) {
  setTimeout(function () {
    console.log("The index of this number is: " + i);
  }, 3000);
}

For ES6, you can just replace var i with let i.

For ES5, you need to create a function scope like here:

var arr = [10, 32, 65, 2];
for (var i = 0; i < arr.length; i++) {
  setTimeout(
    (function (j) {
      return function () {
        console.log("The index of this number is: " + j);
      };
    })(i),
    3000
  );
}

Q. What will be the output of the following code?

console.log(eval("10 + 10")); // 20

console.log(eval("5 + 5" + 10)); // 515

console.log(eval("5 + 5 + 5" + 10)); // 520

console.log(eval(10 + "5 + 5")); // 110

console.log(eval(10 + "5 + 5 + 5")); // 115

Q. What will be the output of the following code?

var x = 10;
var y = 20;
var a = eval("x * y") + "<br>";
var b = eval("2 + 2") + "<br>";
var c = eval("x + 30") + "<br>";

let result = a + b + c;
console.log(result); // 200<br>4<br>40<br>

Q. What will be the output of the following code?

// Example 01:
var prices = [12, 20, 18];
var newPriceArray = [...prices];
console.log(newPriceArray);

// Example 02:
var alphabets = ["A", ..."BCD", "E"];
console.log(alphabets);

// Example 03:
var prices = [12, 20, 18];
var maxPrice = Math.max(...prices);
console.log(maxPrice);

// Example 04:
var max = Math.max(..."43210");
console.log(max);

// Example 05:
const fruits = ["apple", "orange"];
const vegetables = ["carrot", "potato"];

const result = ["bread", ...vegetables, "chicken", ...fruits];
console.log(result);

// Example 06:
const country = "USA";
console.log([...country]);

Q. Given and object and property path. Get value from property path

function getPropertyValue(TEMP_OBJECT, path) {
  return path.split('.').reduce((prev, key) => {
      return prev ? prev[key] : undefined;
    }, TEMP_OBJECT)
}

//Input :
let srcObject = {
    'system' : {
        'database' : {
              '0' : {
                'host' : '54.232.122',
                'port' : 3306
             },
              '1' : {
                'host' : '54.232.123',
             },
             'port' : 3307
              '2' : {
                'host' : '54.232.123',
             }
       }
   }
},
path = "system.database.1.port";

//Output: 3307

Q. How to filter object from Arrays of Objects

let filteredArray = [{name: 'john'},{name: 'kelly'}].filter(value => value.name === 'kelly');

Filter method return Array of objects

Q. How to replace all the occurrences of string

str = str.replace(/test/g, "");

Q. Write a script that returns the number of occurrences of character given a string as input

function countCharacters(str) {
  return str
    .replace(/ /g, "")
    .toLowerCase()
    .split("")
    .reduce((p, c) => {
      if (c in p) {
        p[c]++;
      } else {
        p[c] = 1;
      }
      return p;
    }, {});
}
console.log(countCharacters("the brown fox jumps over the lazy dog"));

Q. write a script that return the number of occurrences of a character in paragraph

function charCount(str, searchChar) {
  let count = 0;
  if (str) {
    let stripStr = str.replace(/ /g, "").toLowerCase(); //remove spaces and covert to lowercase
    for (let chr of stripStr) {
      if (chr === searchChar) {
        count++;
      }
    }
  }
  return count;
}
console.log(charCount("the brown fox jumps over the lazy dog", "o"));

Q. Recursive and non-recursive Factorial function

function recursiveFactorial(n) {
  if (n < 1) {
    throw Error("Value of N has to be greater then 1");
  }
  if (n === 1) {
    return 1;
  } else {
    return n * recursiveFactorial(n - 1);
  }
}

console.log(recursiveFactorial(5));

function factorial(n) {
  if (n < 1) {
    throw Error("Value of N has to be greater then 1");
  }
  if (n === 1) {
    return 1;
  }
  let result = 1;
  for (let i = 1; i <= n; i++) {
    result = result * i;
  }
  return result;
}

console.log(factorial(5));

Q. Recursive and non recursive fibonacci-sequence

// 1, 1, 2, 3, 5, 8, 13, 21, 34

function recursiveFibonacci(num) {
  if (num <= 1) {
    return 1;
  } else {
    return recursiveFibonacci(num - 1) + recursiveFibonacci(num - 2);
  }
}

console.log(recursiveFibonacci(8));

function fibonnaci(num) {
  let a = 1,
    b = 0,
    temp;
  while (num >= 0) {
    temp = a;
    a = a + b;
    b = temp;
    num--;
  }
  return b;
}

console.log(fibonnaci(7));

// Memoization fibonnaci

function fibonnaci(num, memo = {}) {
  if (num in memo) {
    return memo[num];
  }
  if (num <= 1) {
    return 1;
  }
  return (memo[num] = fibonnaci(num - 1, memo) + fibonnaci(num - 2, memo));
}

console.log(fibonnaci(5)); // 8

Q. Random Number between min and max

// 5 to 7
let min = 5;
let max = 7;
console.log(min + Math.floor(Math.random() * (max - min + 1)));

Q. Get HTML form values as JSON object

// Use the array reduce function with form elements.
const formToJSON = (elements) =>
  [].reduce.call(
    elements,
    (data, element) => {
      data[element.name] = element.value;
      // Check if name and value exist on element
      // Check if it checkbox or radio button which can select multiple or single
      //check for multiple select options
      return data;
    },
    {}
  );

// pass the elements to above method, to get values
document.querySelector("HTML_FORM_CLASS").elements;

Q. Reverse the number

function reverse(num) {
  let result = 0;
  while (num != 0) {
    result = result * 10;
    result = result + (num % 10);
    num = Math.floor(num / 10);
  }
  return result;
}

console.log(reverse(12345));

Q. Remove Duplicate elements from Array

var arr = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8];
function removeDuplicate() {
  return ar.reduce((prev, current) => {
    //Cannot use includes of array, since it is not supported by many browser
    if (prev.indexOf(current) === -1) {
      prev.push(current);
    }
    return prev;
  }, []);
}
console.log(removeDuplicate(ar));

const removeDuplicates = (arr) => {
  let holder = {};
  return arr.filter((el) => {
    if (!holder[el]) {
      holder[el] = true;
      return true;
    }
    return false;
  });
};
const arr = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8];
console.log(removeDuplicates(arr)); // ["1", "2", "3", "5", "8", "9"] // O(n)

// Es6
console.log([...new Set(arr)]);

Q. Deep copy of object or clone of object

function deepExtend(out = {}) {
  for (let i = 1; i < arguments.length; i++) {
    let obj = arguments[i];
    if (obj == null)
      // skip undefined and null [check with double equal not triple]
      continue;

    obj = Object(obj);

    for (let key in obj) {
      // avoid shadow hasownproperty of parent
      if (Object.prototype.hasOwnProperty.call(obj, key)) {
        if (
          typeof obj[key] === "object" &&
          !Array.isArray(obj[key]) &&
          obj[key] != null
        )
          out[key] = deepExtend(out[key], obj[key]);
        else out[key] = obj[key];
      }
    }
  }
  return out;
}

//Alternative if there are no function
let cloneObj = JSON.parse(JSON.stringify(obj));

console.log(deepExtend({}, { a: 1, b: { c: 2, d: 3 } }, { e: 4, b: { f: 1 } }));
//output : { a: 1, b: {c: 2, d: 3, f: 1}, e: 4 }

Q. Sort ticket based on flying order.

"use strict";

function SortTickets(tickets) {
  this.tickets = tickets;

  // reverse the order of tickets
  this.reverseTickets = {};
  for (let key in this.tickets) {
    this.reverseTickets[tickets[key]] = key;
  }

  // Get the starting point of ticket
  let orderedTivckets = [...this.getStartingPoint()];

  // Get the ticket destination.
  let currentValue = orderedTickets[orderedTickets.length - 1];
  while (currentValue) {
    currentValue = this.tickets[currentValue];
    if (currentValue) {
      orderedTickets.push(currentValue);
    }
  }
  console.log(orderedTickets);
}

SortTickets.prototype.getStartingPoint = function () {
  for (let tick in this.tickets) {
    if (!(tick in this.reverseTickets)) {
      return [tick, this.tickets[tick]];
    }
  }
  return null;
};

new SortTickets({
  Athens: "Rio",
  Barcelona: "Athens",
  London: "NYC",
  ND: "Lahore",
  NYC: "Barcelona",
  Rio: "ND",
});

Q. Cuncurrent execute function based on input number

function concurrent(num) {
  this.queue = [];
  this.num = num;
}

concurrent.prototype.enqueue = function (value) {
  this.queue.push(value);
};

concurrent.prototype.start = function () {
  this.runningCount = 0;
  while (this.queue.length > 0) {
    if (this.runningCount < this.num) {
      this.queue.pop().call(this, () => {
        this.runningCount--;
        let count = this.runningCount;
        if (count === 0) {
          this.start();
        }
      });
      this.runningCount++;
    }
  }
};

let callback = (done) => {
  console.log("starting");
  setTimeout(() => {
    console.log("stopped");
    done();
  }, 200);
};

let c = new concurrent(2);
c.enqueue(callback);
c.enqueue(callback);
c.enqueue(callback);
c.enqueue(callback);
c.enqueue(callback);
c.enqueue(callback);
c.start();

Q. Reversing an array

let a = [1, 2, 3, 4, 5];

//Approach 1:
console.log(a.reverse());

//Approach 2:
let reverse = a.reduce((prev, current) => {
  prev.unshift(current);
  return prev;
}, []);

console.log(reverse);

Q. Rotate 2D array

const transpose = (arr) => arr[0].map((col, i) => arr.map((row) => row[i]));

console.log(
  transpose([
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
  ])
);

Q. Get Column from 2D Array

const getColumn = (arr, n) => arr.map((x) => x[n]);

const twoDimensionalArray = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

console.log(getColumn(twoDimensionalArray, 1)); //Result = [2,5,8]

Q. Get top N from array

function topN(arr, num) {
  let sorted = arr.sort((a, b) => a - b);
  return sorted.slice(sorted.length - num, sorted.length);
}

console.log(topN([1, 8, 3, 4, 5], 2)); // [5,8]

Q. Get query params from Object

function getQueryParams(obj) {
  let parms = "";
  for (let key in obj) {
    if (obj.hasOwnProperty(key)) {
      if (parms.length > 0) {
        parms += "&";
      }
      parms += encodeURI(`${key}=${obj[key]}`);
    }
  }
  return parms;
}

console.log(
  getQueryParams({
    name: "Umesh",
    tel: "48289",
    add: "3333 emearld st",
  })
);

Q. Consecutive 1’s in binary

function consecutiveOne(num) {
  let binaryArray = num.toString(2);

  let maxOccurence = 0,
    occurence = 0;
  for (let val of binaryArray) {
    if (val === "1") {
      occurence += 1;
      maxOccurence = Math.max(maxOccurence, occurence);
    } else {
      occurence = 0;
    }
  }
  return maxOccurence;
}
//13 = 1101 = 2
//5 = 101 = 1
console.log(consecutiveOne(5)); //1

Q. Spiral travesal of matrix

var input = [
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9, 10, 11, 12],
  [13, 14, 15, 16],
];

var spiralTraversal = function (matriks) {
  let result = [];
  var goAround = function (matrix) {
    if (matrix.length === 0) {
      return;
    }

    // right
    result = result.concat(matrix.shift());

    // down
    for (var j = 0; j < matrix.length - 1; j++) {
      result.push(matrix[j].pop());
    }

    // bottom
    result = result.concat(matrix.pop().reverse());

    // up
    for (var k = matrix.length - 1; k > 0; k--) {
      result.push(matrix[k].shift());
    }

    return goAround(matrix);
  };

  goAround(matriks);

  return result;
};
console.log(spiralTraversal(input)); // [1, 2, 3, 4, 8, 12, 16, 15, 14, 13, 9, 5, 6, 7, 11, 10]

Q. Merge Sorted array and sort it.

function mergeSortedArray(arr1, arr2) {
  return [...new Set(arr1.concat(arr2))].sort((a, b) => a - b);
}

console.log(mergeSortedArray([1, 2, 3, 4, 5, 6], [0, 3, 4, 7])); // [0, 1, 2, 3, 4, 5, 6, 7]

Q. Anagram of words

const alphabetize = (word) => word.split("").sort().join("");

function groupAnagram(wordsArr) {
  return wordsArr.reduce((p, c) => {
    const sortedWord = alphabetize(c);
    if (sortedWord in p) {
      p[sortedWord].push(c);
    } else {
      p[sortedWord] = [c];
    }
    return p;
  }, {});
}

console.log(
  groupAnagram(["map", "art", "how", "rat", "tar", "who", "pam", "shoop"])
);
// result : {
//  amp: ["map", "pam"],
//  art: ["art", "rat", "tar"],
//  hoops: ["shoop"],
//  how: ["how", "who"]
// }

Q. Print the largest (maximum) hourglass sum found in 2d array.

// if arr 6 X 6 then iterate it till 4 X 4  [reduce by two]
// if arr 8 X 8 then iterate it till 6 X 6  [reduce by two]
function main(arr) {
  let maxScore = -999;
  let len = arr.length;
  for (let i = 0; i < len - 2; i++) {
    for (let j = 0; j < len - 2; j++) {
      let total =
        arr[i][j] +
        arr[i][j + 1] +
        arr[i][j + 2] +
        arr[i + 1][j + 1] +
        arr[i + 2][j] +
        arr[i + 2][j + 1] +
        arr[i + 2][j + 2];

      maxScore = Math.max(maxScore, total);
    }
  }
  console.log(maxScore);
}

Q. Transform array of object to array

let data = [
  { vid: "aaa", san: 12 },
  { vid: "aaa", san: 18 },
  { vid: "aaa", san: 2 },
  { vid: "bbb", san: 33 },
  { vid: "bbb", san: 44 },
  { vid: "aaa", san: 100 },
];

let newData = data.reduce((acc, item) => {
  acc[item.vid] = acc[item.vid] || { vid: item.vid, san: [] };
  acc[item.vid]["san"].push(item.san);
  return acc;
}, {});

console.log(Object.keys(newData).map((key) => newData[key]));

// Result
// [[object Object] {
//   san: [12, 18, 2, 100],
//   vid: "aaa"
// }, [object Object] {
//   san: [33, 44],
//   vid: "bbb"
// }]

Q. Create a private variable or private method in object

let obj = (function () {
  function getPrivateFunction() {
    console.log("this is private function");
  }
  let p = "You are accessing private variable";
  return {
    getPrivateProperty: () => {
      console.log(p);
    },
    callPrivateFunction: getPrivateFunction,
  };
})();

obj.getPrivateValue(); // You are accessing private variable
console.log("p" in obj); // false
obj.callPrivateFunction(); // this is private function

Q. Flatten only Array not objects

function flatten(arr, result = []) {
  arr.forEach((val) => {
    if (Array.isArray(val)) {
      flatten(val, result);
    } else {
      result.push(val);
    }
  });
  return result;
}

let input = [1, { a: [2, [3]] }, 4, [5, [6]], [[7, ["hi"]], 8, 9], 10];
console.log(flatten(input)); // [1, { a: [2, [3]]}, 4, 5, 6, 7, "hi", 8, 9, 10]

function flattenIterative(out) {
  // iteratively
  let result = out;
  while (result.some(Array.isArray)) {
    result = [].concat.apply([], result);
  }
  return result;
}
var list1 = [
  [0, 1],
  [2, 3],
  [4, 5],
];
console.log(flattenIterative(list1)); // [0, 1, 2, 3, 4, 5]

function flattenIterative1(current) {
  let result = [];
  while (current.length) {
    let firstValue = current.shift();
    if (Array.isArray(firstValue)) {
      current = firstValue.concat(current);
    } else {
      result.push(firstValue);
    }
  }
  return result;
}

let input = [1, { a: [2, [3]] }, 4, [5, [6]], [[7, ["hi"]], 8, 9], 10];
console.log(flattenIterative1(input));
var list2 = [0, [1, [2, [3, [4, [5]]]]]];
console.log(flattenIterative1(list2)); // [0, 1, 2, 3, 4, 5]

Q. Find max difference between two number in Array

function maxDifference(arr) {
  let maxDiff = 0;

  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      let diff = Math.abs(arr[i] - arr[j]);
      maxDiff = Math.max(maxDiff, diff);
    }
  }
  return maxDiff;
}

console.log(maxDifference([1, 2, 4])); // [1 - 4 ] = 3

Q. swap two number in ES6 [destructing]

let a = 10,
  b = 5;
[a, b] = [b, a];

Q. Panagram ? it means all the 26 letters of alphabet are there

function panagram(input) {
  if (input == null) {
    // Check for null and undefined
    return false;
  }

  if (input.length < 26) {
    // if length is less then 26 then it is not
    return false;
  }
  input = input.replace(/ /g, "").toLowerCase().split("");
  let obj = input.reduce((prev, current) => {
    if (!(current in prev)) {
      prev[current] = current;
    }
    return prev;
  }, {});
  console.log(Object.keys(obj).length === 26 ? "panagram" : "not pangram");
}
processData("We promptly judged antique ivory buckles for the next prize"); // pangram
processData("We promptly judged antique ivory buckles for the prize"); // Not Pangram

Q. Given two identical DOM trees (not the same one), and a node from one of them find the node in the other one.

function indexOf(arrLike, target) {
  return Array.prototype.indexOf.call(arrLike, target);
}

// Given a node and a tree, extract the nodes path
function getPath(root, target) {
  var current = target;
  var path = [];
  while (current !== root) {
    let parentNode = current.parentNode;
    path.unshift(indexOf(parentNode.childNodes, current));
    current = parentNode;
  }
  return path;
}

// Given a tree and a path, let's locate a node
function locateNodeFromPath(node, path) {
  return path.reduce((root, index) => root.childNodes[index], node);
}

const rootA = document.querySelector("#root-a");
const rootB = document.querySelector("#root-b");
const target = rootA.querySelector(".person__age");

console.log(locateNodeFromPath(rootB, getPath(rootA, target)));

Q. Convert a number into a Roman Numeral

function romanize(num) {
  let lookup = {
      M: 1000,
      CM: 900,
      D: 500,
      CD: 400,
      C: 100,
      XC: 90,
      L: 50,
      XL: 40,
      X: 10,
      IX: 9,
      V: 5,
      IV: 4,
      I: 1,
    },
    roman = "";
  for (let i in lookup) {
    while (num >= lookup[i]) {
      roman += i;
      num -= lookup[i];
    }
  }
  return roman;
}

console.log(romanize(3)); // III

Q. check if parenthesis is malformed or not

function matchParenthesis(str) {
  let obj = { "{": "}", "(": ")", "[": "]" };
  let result = [];
  for (let s of str) {
    if (s === "{" || s === "(" || s === "[") {
      // All opening brackets
      result.push(s);
    } else {
      if (result.length > 0) {
        let lastValue = result.pop(); //pop the last value and compare with key
        if (obj[lastValue] !== s) {
          // if it is not same then it is not formated properly
          return false;
        }
      } else {
        return false; // empty array, there is nothing to pop. so it is not formated properly
      }
    }
  }
  return result.length === 0;
}

console.log(matchParenthesis("}{{}}"), matchParenthesis("{{[]}}")); // false - true

Q. Create Custom Event Emitter class

class EventEmitter {
  constructor() {
    this.holder = {};
  }

  on(eventName, fn) {
    if (eventName && typeof fn === "function") {
      this.holder[eventName] = this.holder[eventName] || [];
      this.holder[eventName].push(fn);
    }
  }

  emit(eventName, ...args) {
    let eventColl = this.holder[eventName];
    if (eventColl) {
      eventColl.forEach((callback) => callback(args));
    }
  }
}

let e = new EventEmitter();
e.on("callme", function (args) {
  console.log(`you called me ${args}`);
});
e.on("callme", function (args) {
  console.log(`testing`);
});

e.emit("callme", ["a", "b"], { firstName: "umesh", lastName: "gohil" });

Q. Max value from an array

const arr = [-2, -3, 4, 3, 2, 1];
Math.max(...arr); // Fastest

Math.max.apply(Math, arr); // Slow

Q. DOM methods

https://github.com/nefe/You-Dont-Need-jQuery

var el = document.querySelector('div');
el.childNodes;   // get the list of child nodes of el
el.firstChild;   // get the first child node of el
el.lastChild;    // get the last child node of el
el.parentNode;   // get the parent node of el
el.previousSibling;    // get the previous sibling of el
el.nextSibling;  // get the next sibling of el
el.textContent;  // get the text content of el
el.innerHTML;    // get the inner html of el

document.createElement('div')  // create dom element
document.creatTextNode('Hello world');  // create text node
document.createDocumentFragment();

el.appendChild(); //append child to el;
el.insertBefore(); // insert child before el;
el.parentNode.replaceChild(NEW_NODE, REPLACE_ME)  // replace the node
el.removechild();  // remove the child node

Array.from(NODES) // convert nodelist to regular array

el.classList[contains | add | remove | replace]  // class of el

el.dataset.<camelCaseName> // data-count is dataset.count, data-index-number is dataset.indexNumber

el.setAttribute | el.getAttribute | el.removeAttribute // attributes of el

el.style    // get the style of el

Q. search function called after 500 ms

<input type="text" class="search" />;

let timer = null;
function searchOptions(value) {
  clearTimeout(timer);
  timer = setTimeout(() => {
    console.log(`value is - ${value}`);
  }, 500);
}

let search = document.querySelector(".search");
search.addEventListener("keyup", function () {
  searchOptions(this.value);
});

Q. Move all zero’s to end

const moveZeroToEnd = (arr) => {
  for (let i = 0, j = 0; j < arr.length; j++) {
    if (arr[j] !== 0) {
      if (i < j) {
        [arr[i], arr[j]] = [arr[j], arr[i]]; // swap i and j
      }
      i++;
    }
  }
  return arr;
};

console.log(moveZeroToEnd([1, 8, 2, 0, 0, 0, 3, 4, 0, 5, 0])); // [1, 8, 2, 3, 4, 5, 0, 0, 0, 0, 0]

Q. Decode message in matrix [diagional down right, diagional up right]

const decodeMessage = (mat) => {
  // check if matrix is null or empty
  if (mat == null || mat.length === 0) {
    return "";
  }
  let x = mat.length - 1;
  let y = mat[0].length - 1;
  let message = "";
  let decode = (mat, i = 0, j = 0, direction = "DOWN") => {
    message += mat[i][j];

    if (i === x) {
      direction = "UP";
    }

    if (direction === "DOWN") {
      i++;
    } else {
      i--;
    }

    if (j === y) {
      return;
    }

    j++;
    decode(mat, i, j, direction);
  };
  decode(mat);
  return message;
};

let mat = [
  ["I", "B", "C", "A", "L", "K", "A"],
  ["D", "R", "F", "C", "A", "E", "A"],
  ["G", "H", "O", "E", "L", "A", "D"],
  ["G", "H", "O", "E", "L", "A", "D"],
];

console.log(decodeMessage(mat)); //IROELEA

Q. find a pair in array, whose sum is equal to given number.

const hasPairSum = (arr, sum) => {
  if (arr == null && arr.length < 2) {
    return false;
  }

  let left = 0;
  let right = arr.length - 1;
  let result = false;

  while (left < right && !result) {
    let pairSum = arr[left] + arr[right];
    if (pairSum < sum) {
      left++;
    } else if (pairSum > sum) {
      right--;
    } else {
      result = true;
    }
  }
  return result;
};

console.log(hasPairSum([1, 2, 4, 5], 8)); // null
console.log(hasPairSum([1, 2, 4, 4], 8)); // [2,3]

const hasPairSum = (arr, sum) => {
  let difference = {};
  let hasPair = false;
  arr.forEach((item) => {
    let diff = sum - item;
    if (!difference[diff]) {
      difference[item] = true;
    } else {
      hasPair = true;
    }
  });
  return hasPair;
};
console.log(hasPairSum([6, 4, 3, 8], 8));

// NOTE: if array is not sorted then subtract the value with sum and store in difference
// then see if that value exist in difference then return true.

Q. Binary Search [Array should be sorted]

function binarySearch(arr, val) {
  let startIndex = 0,
    stopIndex = arr.length - 1,
    middleIndex = Math.floor((startIndex + stopIndex) / 2);

  while (arr[middleIndex] !== val && startIndex < stopIndex) {
    if (val < arr[middleIndex]) {
      stopIndex = middleIndex - 1;
    } else if (val > arr[middleIndex]) {
      startIndex = middleIndex + 1;
    }
    middleIndex = Math.floor((startIndex + stopIndex) / 2);
  }

  return arr[middleIndex] === val ? middleIndex : -1;
}

console.log(binarySearch([-1, 10, 22, 35, 48, 56, 67], 22));
console.log(binarySearch([-1, 10, 22, 35, 48, 56, 67], 27));

Q. Pascal triangle.

function pascalTriangle(n) {
  let last = [1],
    triangle = [last];
  for (let i = 0; i < n; i++) {
    const ls = [0].concat(last), //[0,1]           // [0,1,1]
      rs = last.concat([0]); //[1,0]           // [1,1,0]
    last = rs.map((r, i) => ls[i] + r); //[1, 1]          // [1,2,1]
    triangle = triangle.concat([last]); // [[1], [1,1]]   // [1], [1, 1], [1, 2, 1]
  }
  return triangle;
}

console.log(pascalTriangle(2));

Q. Explain the code below. How many times the createVal function is called?

function createVal() {
  return Math.random();
}

function fun(val = createVal()) {
  console.log(val);
}

fun();
fun(5);

createVal() function will execute only once.

Output

0.2162050091554224
VM298:6 5

Q. What is the output?

function sayHi() {
  console.log(name);
  console.log(age);
  var name = "Lydia";
  let age = 21;
}

sayHi();
  • A: Lydia and undefined
  • B: Lydia and ReferenceError
  • C: ReferenceError and 21
  • D: undefined and ReferenceError

Answer: D

Within the function, we first declare the name variable with the var keyword. This means that the variable gets hoisted (memory space is set up during the creation phase) with the default value of undefined, until we actually get to the line where we define the variable. We haven’t defined the variable yet on the line where we try to log the name variable, so it still holds the value of undefined.

Variables with the let keyword (and const) are hoisted, but unlike var, don’t get initialized. They are not accessible before the line we declare (initialize) them. This is called the “temporal dead zone”. When we try to access the variables before they are declared, JavaScript throws a ReferenceError.

Q. What is the output?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}
  • A: 0 1 2 and 0 1 2
  • B: 0 1 2 and 3 3 3
  • C: 3 3 3 and 0 1 2

Answer: C

Because of the event queue in JavaScript, the setTimeout callback function is called after the loop has been executed. Since the variable i in the first loop was declared using the var keyword, this value was global. During the loop, we incremented the value of i by 1 each time, using the unary operator ++. By the time the setTimeout callback function was invoked, i was equal to 3 in the first example.

In the second loop, the variable i was declared using the let keyword: variables declared with the let (and const) keyword are block-scoped (a block is anything between { }). During each iteration, i will have a new value, and each value is scoped inside the loop.

Q. What is the output?

const shape = {
  radius: 10,
  diameter() {
    return this.radius * 2;
  },
  perimeter: () => 2 * Math.PI * this.radius,
};

console.log(shape.diameter());
console.log(shape.perimeter());
  • A: 20 and 62.83185307179586
  • B: 20 and NaN
  • C: 20 and 63
  • D: NaN and 63

Answer: B

Note that the value of diameter is a regular function, whereas the value of perimeter is an arrow function.

With arrow functions, the this keyword refers to its current surrounding scope, unlike regular functions! This means that when we call perimeter, it doesn’t refer to the shape object, but to its surrounding scope (window for example).

There is no value radius on that object, which returns undefined.

Q. What is the output?

+true;
!"Lydia";
  • A: 1 and false
  • B: false and NaN
  • C: false and false

Answer: A

The unary plus tries to convert an operand to a number. true is 1, and false is 0.

The string 'Lydia' is a truthy value. What we’re actually asking, is “is this truthy value falsy?”. This returns false.

Q. Which one is true?

const bird = {
  size: "small",
};

const mouse = {
  name: "Mickey",
  small: true,
};
  • A: mouse.bird.size is not valid
  • B: mouse[bird.size] is not valid
  • C: mouse[bird["size"]] is not valid
  • D: All of them are valid

Answer: A

In JavaScript, all object keys are strings (unless it’s a Symbol). Even though we might not type them as strings, they are always converted into strings under the hood.

JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [ and keeps going until it finds the closing bracket ]. Only then, it will evaluate the statement.

mouse[bird.size]: First it evaluates bird.size, which is "small"mouse["small"] returns true

However, with dot notation, this doesn’t happen. mouse does not have a key called bird, which means that mouse.bird is undefined. Then, we ask for the size using dot notation: mouse.bird.size. Since mouse.bird is undefined, we’re actually asking undefined.size. This isn’t valid, and will throw an error similar to Cannot read property "size" of undefined.

Q. What is the output?

let c = { greeting: "Hey!" };
let d;

d = c;
c.greeting = "Hello";
console.log(d.greeting);
  • A: Hello
  • B: Hey!
  • C: undefined
  • D: ReferenceError
  • E: TypeError

Answer: A

In JavaScript, all objects interact by reference when setting them equal to each other.

First, variable c holds a value to an object. Later, we assign d with the same reference that c has to the object.

jj

When you change one object, you change all of them.

Q. What is the output?

let a = 3;
let b = new Number(3);
let c = 3;

console.log(a == b);
console.log(a === b);
console.log(b === c);
  • A: true false true
  • B: false false true
  • C: true false false
  • D: false true true

Answer: C

new Number() is a built-in function constructor. Although it looks like a number, it’s not really a number: it has a bunch of extra features and is an object.

When we use the == operator, it only checks whether it has the same value. They both have the value of 3, so it returns true.

However, when we use the === operator, both value and type should be the same. It’s not: new Number() is not a number, it’s an object. Both return false.

Q. What is the output?

class Chameleon {
  static colorChange(newColor) {
    this.newColor = newColor;
    return this.newColor;
  }

  constructor({ newColor = "green" } = {}) {
    this.newColor = newColor;
  }
}

const freddie = new Chameleon({ newColor: "purple" });
console.log(freddie.colorChange("orange"));
  • A: orange
  • B: purple
  • C: green
  • D: TypeError

Answer: D

The colorChange function is static. Static methods are designed to live only on the constructor in which they are created, and cannot be passed down to any children. Since freddie is a child, the function is not passed down, and not available on the freddie instance: a TypeError is thrown.

Q. What is the output?

let greeting;
greetign = {}; // Typo!
console.log(greetign);
  • A: {}
  • B: ReferenceError: greetign is not defined
  • C: undefined

Answer: A

It logs the object, because we just created an empty object on the global object! When we mistyped greeting as greetign, the JS interpreter actually saw this as global.greetign = {} (or window.greetign = {} in a browser).

In order to avoid this, we can use "use strict". This makes sure that you have declared a variable before setting it equal to anything.

Q. What happens when we do this?

function bark() {
  console.log("Woof!");
}

bark.animal = "dog";
  • A: Nothing, this is totally fine!
  • B: SyntaxError. You cannot add properties to a function this way.
  • C: "Woof" gets logged.
  • D: ReferenceError

Answer: A

This is possible in JavaScript, because functions are objects! (Everything besides primitive types are objects)

A function is a special type of object. The code you write yourself isn’t the actual function. The function is an object with properties. This property is invocable.

Q. What is the output?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const member = new Person("Lydia", "Hallie");
Person.getFullName = function () {
  return `${this.firstName} ${this.lastName}`;
};

console.log(member.getFullName());
  • A: TypeError
  • B: SyntaxError
  • C: Lydia Hallie
  • D: undefined undefined

Answer: A

You can’t add properties to a constructor like you can with regular objects. If you want to add a feature to all objects at once, you have to use the prototype instead. So in this case,

Person.prototype.getFullName = function () {
  return `${this.firstName} ${this.lastName}`;
};

would have made member.getFullName() work. Why is this beneficial? Say that we added this method to the constructor itself. Maybe not every Person instance needed this method. This would waste a lot of memory space, since they would still have that property, which takes of memory space for each instance. Instead, if we only add it to the prototype, we just have it at one spot in memory, yet they all have access to it!

Q. What is the output?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const lydia = new Person("Lydia", "Hallie");
const sarah = Person("Sarah", "Smith");

console.log(lydia);
console.log(sarah);
  • A: Person {firstName: "Lydia", lastName: "Hallie"} and undefined
  • B: Person {firstName: "Lydia", lastName: "Hallie"} and Person {firstName: "Sarah", lastName: "Smith"}
  • C: Person {firstName: "Lydia", lastName: "Hallie"} and {}
  • D:Person {firstName: "Lydia", lastName: "Hallie"} and ReferenceError

Answer: A

For sarah, we didn’t use the new keyword. When using new, it refers to the new empty object we create. However, if you don’t add new it refers to the global object!

We said that this.firstName equals "Sarah" and this.lastName equals "Smith". What we actually did, is defining global.firstName = 'Sarah' and global.lastName = 'Smith'sarah itself is left undefined, since we don’t return a value from the Person function.

Q. What are the three phases of event propagation?

  • A: Target > Capturing > Bubbling
  • B: Bubbling > Target > Capturing
  • C: Target > Bubbling > Capturing
  • D: Capturing > Target > Bubbling

Answer: D

During the capturing phase, the event goes through the ancestor elements down to the target element. It then reaches the target element, and bubbling begins.

h

Q. All object have prototypes.

  • A: true
  • B: false

Answer: B

All objects have prototypes, except for the base object. The base object is the object created by the user, or an object that is created using the new keyword. The base object has access to some methods and properties, such as .toString. This is the reason why you can use built-in JavaScript methods! All of such methods are available on the prototype. Although JavaScript can’t find it directly on your object, it goes down the prototype chain and finds it there, which makes it accessible for you.

Q. What is the output?

function sum(a, b) {
  return a + b;
}

sum(1, "2");
  • A: NaN
  • B: TypeError
  • C: "12"
  • D: 3

Answer: C

JavaScript is a dynamically typed language: we don’t specify what types certain variables are. Values can automatically be converted into another type without you knowing, which is called implicit type coercionCoercion is converting from one type into another.

In this example, JavaScript converts the number 1 into a string, in order for the function to make sense and return a value. During the addition of a numeric type (1) and a string type ('2'), the number is treated as a string. We can concatenate strings like "Hello" + "World", so What is happening here is "1" + "2" which returns "12".

Q. What is the output?

let number = 0;
console.log(number++);
console.log(++number);
console.log(number);
  • A: 1 1 2
  • B: 1 2 2
  • C: 0 2 2
  • D: 0 1 2

Answer: C

The postfix unary operator ++:

  1. Returns the value (this returns 0)
  2. Increments the value (number is now 1)

The prefix unary operator ++:

  1. Increments the value (number is now 2)
  2. Returns the value (this returns 2)

This returns 0 2 2.

Q. What is the output?

function getPersonInfo(one, two, three) {
  console.log(one);
  console.log(two);
  console.log(three);
}

const person = "Lydia";
const age = 21;

getPersonInfo`${person} is ${age} years old`;
  • A: "Lydia" 21 ["", " is ", " years old"]
  • B: ["", " is ", " years old"] "Lydia" 21
  • C: "Lydia" ["", " is ", " years old"] 21

Answer: B

If you use tagged template literals, the value of the first argument is always an array of the string values. The remaining arguments get the values of the passed expressions!

Q. What is the output?

function checkAge(data) {
  if (data === { age: 18 }) {
    console.log("You are an adult!");
  } else if (data == { age: 18 }) {
    console.log("You are still an adult.");
  } else {
    console.log(`Hmm.. You don't have an age I guess`);
  }
}

checkAge({ age: 18 });
  • A: You are an adult!
  • B: You are still an adult.
  • C: Hmm.. You don't have an age I guess

Answer: C

When testing equality, primitives are compared by their value, while objects are compared by their reference. JavaScript checks if the objects have a reference to the same location in memory.

The two objects that we are comparing don’t have that: the object we passed as a parameter refers to a different location in memory than the object we used in order to check equality.

This is why both { age: 18 } === { age: 18 } and { age: 18 } == { age: 18 } return false.

Q. What is the output?

function getAge(...args) {
  console.log(typeof args);
}

getAge(21);
  • A: "number"
  • B: "array"
  • C: "object"
  • D: "NaN"

Answer: C

The rest parameter (...args.) lets us “collect” all remaining arguments into an array. An array is an object, so typeof args returns "object"

Q. What is the output?

function getAge() {
  "use strict";
  age = 21;
  console.log(age);
}

getAge();
  • A: 21
  • B: undefined
  • C: ReferenceError
  • D: TypeError

Answer: C

With "use strict", you can make sure that you don’t accidentally declare global variables. We never declared the variable age, and since we use "use strict", it will throw a reference error. If we didn’t use "use strict", it would have worked, since the property age would have gotten added to the global object.

Q. What is value of sum?

const sum = eval("10*10+5");
  • A: 105
  • B: "105"
  • C: TypeError
  • D: "10*10+5"

Answer: A

eval evaluates codes that’s passed as a string. If it’s an expression, like in this case, it evaluates the expression. The expression is 10 * 10 + 5. This returns the number 105.

Q. How long is cool_secret accessible?

sessionStorage.setItem("cool_secret", 123);
  • A: Forever, the data doesn’t get lost.
  • B: When the user closes the tab.
  • C: When the user closes the entire browser, not only the tab.
  • D: When the user shuts off their computer.

Answer: B

The data stored in sessionStorage is removed after closing the tab.

If you used localStorage, the data would’ve been there forever, unless for example localStorage.clear() is invoked.

Q. What is the output?

var num = 8;
var num = 10;

console.log(num);
  • A: 8
  • B: 10
  • C: SyntaxError
  • D: ReferenceError

Answer: B

With the var keyword, you can declare multiple variables with the same name. The variable will then hold the latest value. You cannot do this with let or const since they’re block-scoped.

Q. What is the output?

const obj = { 1: "a", 2: "b", 3: "c" };
const set = new Set([1, 2, 3, 4, 5]);

obj.hasOwnProperty("1");
obj.hasOwnProperty(1);
set.has("1");
set.has(1);
  • A: false true false true
  • B: false true true true
  • C: true true false true
  • D: true true true true

Answer: C

All object keys (excluding Symbols) are strings under the hood, even if you don’t type it yourself as a string. This is why obj.hasOwnProperty('1') also returns true.

It doesn’t work that way for a set. There is no '1' in our set: set.has('1') returns false. It has the numeric type 1set.has(1) returns true.

Q. What is the output?

const obj = { a: "one", b: "two", a: "three" };
console.log(obj);
  • A: { a: "one", b: "two" }
  • B: { b: "two", a: "three" }
  • C: { a: "three", b: "two" }
  • D: SyntaxError

Answer: C

If you have two keys with the same name, the key will be replaced. It will still be in its first position, but with the last specified value.

Q. The JavaScript global execution context creates two things for you: the global object, and the “this” keyword.

  • A: true
  • B: false
  • C: it depends

Answer: A

The base execution context is the global execution context: it’s What is accessible everywhere in your code.

Q. What is the output?

for (let i = 1; i < 5; i++) {
  if (i === 3) continue;
  console.log(i);
}
  • A: 1 2
  • B: 1 2 3
  • C: 1 2 4
  • D: 1 3 4

Answer: C

The continue statement skips an iteration if a certain condition returns true.

Q. What is the output?

String.prototype.giveLydiaPizza = () => {
  return "Just give Lydia pizza already!";
};

const name = "Lydia";

name.giveLydiaPizza();
  • A: "Just give Lydia pizza already!"
  • B: TypeError: not a function
  • C: SyntaxError
  • D: undefined

Answer: A

String is a built-in constructor, which we can add properties to. I just added a method to its prototype. Primitive strings are automatically converted into a string object, generated by the string prototype function. So, all strings (string objects) have access to that method!

Q. What is the output?

const a = {};
const b = { key: "b" };
const c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);
  • A: 123
  • B: 456
  • C: undefined
  • D: ReferenceError

Answer: B

Object keys are automatically converted into strings. We are trying to set an object as a key to object a, with the value of 123.

However, when we stringify an object, it becomes "[Object object]". So what we are saying here, is that a["Object object"] = 123. Then, we can try to do the same again. c is another object that we are implicitly stringifying. So then, a["Object object"] = 456.

Then, we log a[b], which is actually a["Object object"]. We just set that to 456, so it returns 456.

Q. What is the output?

const foo = () => console.log("First");
const bar = () => setTimeout(() => console.log("Second"));
const baz = () => console.log("Third");

bar();
foo();
baz();
  • A: First Second Third
  • B: First Third Second
  • C: Second First Third
  • D: Second Third First

Answer: B

We have a setTimeout function and invoked it first. Yet, it was logged last.

This is because in browsers, we don’t just have the runtime engine, we also have something called a WebAPI. The WebAPI gives us the setTimeout function to start with, and for example the DOM.

After the callback is pushed to the WebAPI, the setTimeout function itself (but not the callback!) is popped off the stack.

a

Now, foo gets invoked, and "First" is being logged.

foo is popped off the stack, and baz gets invoked. "Third" gets logged.

c

The WebAPI can’t just add stuff to the stack whenever it’s ready. Instead, it pushes the callback function to something called the queue.

d

This is where an event loop starts to work. An event loop looks at the stack and task queue. If the stack is empty, it takes the first thing on the queue and pushes it onto the stack.

e

bar gets invoked, "Second" gets logged, and it’s popped off the stack.

Q. What is the event.target when clicking the button?

<div onclick="console.log('first div')">
  <div onclick="console.log('second div')">
    <button onclick="console.log('button')">Click!</button>
  </div>
</div>
  • A: Outer div
  • B: Inner div
  • C: button
  • D: An array of all nested elements.

Answer: C

The deepest nested element that caused the event is the target of the event. You can stop bubbling by event.stopPropagation

Q. When you click the paragraph, What is the logged output?

<div onclick="console.log('div')">
  <p onclick="console.log('p')">Click here!</p>
</div>
  • A: p div
  • B: div p
  • C: p
  • D: div

Answer: A

If we click p, we see two logs: p and div. During event propagation, there are 3 phases: capturing, target, and bubbling. By default, event handlers are executed in the bubbling phase (unless you set useCapture to true). It goes from the deepest nested element outwards.

Q. What is the output?

const person = { name: "Lydia" };

function sayHi(age) {
  console.log(`${this.name} is ${age}`);
}

sayHi.call(person, 21);
sayHi.bind(person, 21);
  • A: undefined is 21 Lydia is 21
  • B: function function
  • C: Lydia is 21 Lydia is 21
  • D: Lydia is 21 function

Answer: D

With both, we can pass the object to which we want the this keyword to refer to. However, .call is also executed immediately!

.bind. returns a copy of the function, but with a bound context! It is not executed immediately.

Q. What is the output?

function sayHi() {
  return (() => 0)();
}

console.log(typeof sayHi());
  • A: "object"
  • B: "number"
  • C: "function"
  • D: "undefined"

Answer: B

The sayHi function returns the returned value of the immediately invoked function (IIFE). This function returned 0, which is type "number".

FYI: there are only 7 built-in types: nullundefinedbooleannumberstringobject, and symbol"function" is not a type, since functions are objects, it’s of type "object".

Q. Which of these values are falsy?

0;
new Number(0);
("");
(" ");
new Boolean(false);
undefined;
  • A: 0''undefined
  • B: 0new Number(0)''new Boolean(false)undefined
  • C: 0''new Boolean(false)undefined
  • D: All of them are falsy

Answer: A

There are only six falsy values:

  • undefined
  • null
  • NaN
  • 0
  • '' (empty string)
  • false

Function constructors, like new Number and new Boolean are truthy.

Q. What is the output?

console.log(typeof typeof 1);
  • A: "number"
  • B: "string"
  • C: "object"
  • D: "undefined"

Answer: B

typeof 1 returns "number"typeof "number" returns "string"

Q. What is the output?

const numbers = [1, 2, 3];
numbers[10] = 11;
console.log(numbers);
  • A: [1, 2, 3, 7 x null, 11]
  • B: [1, 2, 3, 11]
  • C: [1, 2, 3, 7 x empty, 11]
  • D: SyntaxError

Answer: C

When you set a value to an element in an array that exceeds the length of the array, JavaScript creates something called “empty slots”. These actually have the value of undefined, but you will see something like:

[1, 2, 3, 7 x empty, 11]

depending on where you run it (it’s different for every browser, node, etc.)

Q. What is the output?

(() => {
  let x, y;
  try {
    throw new Error();
  } catch (x) {
    (x = 1), (y = 2);
    console.log(x);
  }
  console.log(x);
  console.log(y);
})();
  • A: 1 undefined 2
  • B: undefined undefined undefined
  • C: 1 1 2
  • D: 1 undefined undefined

Answer: A

The catch block receives the argument x. This is not the same x as the variable when we pass arguments. This variable x is block-scoped.

Later, we set this block-scoped variable equal to 1, and set the value of the variable y. Now, we log the block-scoped variable x, which is equal to 1.

Outside of the catch block, x is still undefined, and y is 2. When we want to console.log(x) outside of the catch block, it returns undefined, and y returns 2.

Q. Everything in JavaScript is either a…

  • A: primitive or object
  • B: function or object
  • C: trick question! only objects
  • D: number or object

Answer: A

JavaScript only has primitive types and objects.

Primitive types are booleannullundefinedbigintnumberstring, and symbol.

What differentiates a primitive from an object is that primitives do not have any properties or methods; however, you’ll note that 'foo'.toUpperCase() evaluates to 'FOO' and does not result in a TypeError. This is because when you try to access a property or method on a primitive like a string, JavaScript will implicitly wrap the object using one of the wrapper classes, i.e. String, and then immediately discard the wrapper after the expression evaluates. All primitives except for null and undefined exhibit this behaviour.

Q. What is the output?

[
  [0, 1],
  [2, 3],
].reduce(
  (acc, cur) => {
    return acc.concat(cur);
  },
  [1, 2]
);
  • A: [0, 1, 2, 3, 1, 2]
  • B: [6, 1, 2]
  • C: [1, 2, 0, 1, 2, 3]
  • D: [1, 2, 6]

Answer: C

[1, 2] is our initial value. This is the value we start with, and the value of the very first acc. During the first round, acc is [1,2], and cur is [0, 1]. We concatenate them, which results in [1, 2, 0, 1].

Then, [1, 2, 0, 1] is acc and [2, 3] is cur. We concatenate them, and get [1, 2, 0, 1, 2, 3]

Q. What is the output?

!!null;
!!"";
!!1;
  • A: false true false
  • B: false false true
  • C: false true true
  • D: true true false

Answer: B

null is falsy. !null returns true!true returns false.

"" is falsy. !"" returns true!true returns false.

1 is truthy. !1 returns false!false returns true.

Q. What does the setInterval method return in the browser?

setInterval(() => console.log("Hi"), 1000);
  • A: a unique id
  • B: the amount of milliseconds specified
  • C: the passed function
  • D: undefined

Answer: A

It returns a unique id. This id can be used to clear that interval with the clearInterval() function.

Q. What does this return?

[..."Lydia"];
  • A: ["L", "y", "d", "i", "a"]
  • B: ["Lydia"]
  • C: [[], "Lydia"]
  • D: [["L", "y", "d", "i", "a"]]

Answer: A

A string is an iterable. The spread operator maps every character of an iterable to one element.

Q. What is the output?

function* generator(i) {
  yield i;
  yield i * 2;
}

const gen = generator(10);

console.log(gen.next().value);
console.log(gen.next().value);
  • A: [0, 10], [10, 20]
  • B: 20, 20
  • C: 10, 20
  • D: 0, 10 and 10, 20

Answer: C

Regular functions cannot be stopped mid-way after invocation. However, a generator function can be “stopped” midway, and later continue from where it stopped. Every time a generator function encounters a yield keyword, the function yields the value specified after it. Note that the generator function in that case doesn’t return the value, it yields the value.

First, we initialize the generator function with i equal to 10. We invoke the generator function using the next() method. The first time we invoke the generator function, i is equal to 10. It encounters the first yield keyword: it yields the value of i. The generator is now “paused”, and 10 gets logged.

Then, we invoke the function again with the next() method. It starts to continue where it stopped previously, still with i equal to 10. Now, it encounters the next yield keyword, and yields i * 2i is equal to 10, so it returns 10 * 2, which is 20. This results in 10, 20.

Q. What does this return?

const firstPromise = new Promise((res, rej) => {
  setTimeout(res, 500, "one");
});

const secondPromise = new Promise((res, rej) => {
  setTimeout(res, 100, "two");
});

Promise.race([firstPromise, secondPromise]).then((res) => console.log(res));
  • A: "one"
  • B: "two"
  • C: "two" "one"
  • D: "one" "two"

Answer: B

When we pass multiple promises to the Promise.race method, it resolves/rejects the first promise that resolves/rejects. To the setTimeout method, we pass a timer: 500ms for the first promise (firstPromise), and 100ms for the second promise (secondPromise). This means that the secondPromise resolves first with the value of 'two'res now holds the value of 'two', which gets logged.

Q. What is the output?

let person = { name: "Lydia" };
const members = [person];
person = null;

console.log(members);
  • A: null
  • B: [null]
  • C: [{}]
  • D: [{ name: "Lydia" }]

Answer: D

First, we declare a variable person with the value of an object that has a name property.

p

Then, we declare a variable called members. We set the first element of that array equal to the value of the person variable. Objects interact by reference when setting them equal to each other. When you assign a reference from one variable to another, you make a copy of that reference. (note that they don’t have the same reference!)

p2

Then, we set the variable person equal to null.

We are only modifying the value of the person variable, and not the first element in the array, since that element has a different (copied) reference to the object. The first element in members still holds its reference to the original object. When we log the members array, the first element still holds the value of the object, which gets logged.

Q. What is the output?

const person = {
  name: "Lydia",
  age: 21,
};

for (const item in person) {
  console.log(item);
}
  • A: { name: "Lydia" }, { age: 21 }
  • B: "name", "age"
  • C: "Lydia", 21
  • D: ["name", "Lydia"], ["age", 21]

Answer: B

With a for-in loop, we can iterate through object keys, in this case name and age. Under the hood, object keys are strings (if they’re not a Symbol). On every loop, we set the value of item equal to the current key it’s iterating over. First, item is equal to name, and gets logged. Then, item is equal to age, which gets logged.

Q. What is the output?

console.log(3 + 4 + "5");
  • A: "345"
  • B: "75"
  • C: 12
  • D: "12"

Answer: B

Operator associativity is the order in which the compiler evaluates the expressions, either left-to-right or right-to-left. This only happens if all operators have the same precedence. We only have one type of operator: +. For addition, the associativity is left-to-right.

3 + 4 gets evaluated first. This results in the number 7.

7 + '5' results in "75" because of coercion. JavaScript converts the number 7 into a string, see question 15. We can concatenate two strings using the +operator. "7" + "5" results in "75".

Q. What is the value of num?

const num = parseInt("7*6", 10);
  • A: 42
  • B: "42"
  • C: 7
  • D: NaN

Answer: C

Only the first numbers in the string is returned. Based on the radix (the second argument in order to specify what type of number we want to parse it to: base 10, hexadecimal, octal, binary, etc.), the parseInt checks whether the characters in the string are valid. Once it encounters a character that isn’t a valid number in the radix, it stops parsing and ignores the following characters.

* is not a valid number. It only parses "7" into the decimal 7num now holds the value of 7.

Q. What is the output`?

[1, 2, 3].map((num) => {
  if (typeof num === "number") return;
  return num * 2;
});
  • A: []
  • B: [null, null, null]
  • C: [undefined, undefined, undefined]
  • D: [ 3 x empty ]

Answer: C

When mapping over the array, the value of num is equal to the element it’s currently looping over. In this case, the elements are numbers, so the condition of the if statement typeof num === "number" returns true. The map function creates a new array and inserts the values returned from the function.

However, we don’t return a value. When we don’t return a value from the function, the function returns undefined. For every element in the array, the function block gets called, so for each element we return undefined.

Q. What is the output?

function getInfo(member, year) {
  member.name = "Lydia";
  year = "1998";
}

const person = { name: "Sarah" };
const birthYear = "1997";

getInfo(person, birthYear);

console.log(person, birthYear);
  • A: { name: "Lydia" }, "1997"
  • B: { name: "Sarah" }, "1998"
  • C: { name: "Lydia" }, "1998"
  • D: { name: "Sarah" }, "1997"

Answer: A

Arguments are passed by value, unless their value is an object, then they’re passed by referencebirthYear is passed by value, since it’s a string, not an object. When we pass arguments by value, a copy of that value is created (see question 46).

The variable birthYear has a reference to the value "1997". The argument year also has a reference to the value "1997", but it’s not the same value as birthYear has a reference to. When we update the value of year by setting year equal to "1998", we are only updating the value of yearbirthYear is still equal to "1997".

The value of person is an object. The argument member has a (copied) reference to the same object. When we modify a property of the object member has a reference to, the value of person will also be modified, since they both have a reference to the same object. person‘s name property is now equal to the value "Lydia"

Q. What is the output?

function greeting() {
  throw "Hello world!";
}

function sayHi() {
  try {
    const data = greeting();
    console.log("It worked!", data);
  } catch (e) {
    console.log("Oh no an error:", e);
  }
}

sayHi();
  • A: It worked! Hello world!
  • B: Oh no an error: undefined
  • C: SyntaxError: can only throw Error objects
  • D: Oh no an error: Hello world!

Answer: D

With the throw statement, we can create custom errors. With this statement, you can throw exceptions. An exception can be a string, a number, a boolean or an object. In this case, our exception is the string 'Hello world'.

With the catch statement, we can specify what to do if an exception is thrown in the try block. An exception is thrown: the string 'Hello world'e is now equal to that string, which we log. This results in 'Oh an error: Hello world'.

Q. What is the output?

function Car() {
  this.make = "Lamborghini";
  return { make: "Maserati" };
}

const myCar = new Car();
console.log(myCar.make);
  • A: "Lamborghini"
  • B: "Maserati"
  • C: ReferenceError
  • D: TypeError

Answer: B

When you return a property, the value of the property is equal to the returned value, not the value set in the constructor function. We return the string "Maserati", so myCar.make is equal to "Maserati".

Q. What is the output?

(() => {
  let x = (y = 10);
})();

console.log(typeof x);
console.log(typeof y);
  • A: "undefined", "number"
  • B: "number", "number"
  • C: "object", "number"
  • D: "number", "undefined"

Answer: A

let x = y = 10; is actually shorthand for:

y = 10;
let x = y;

When we set y equal to 10, we actually add a property y to the global object (window in browser, global in Node). In a browser, window.y is now equal to 10.

Then, we declare a variable x with the value of y, which is 10. Variables declared with the let keyword are block scoped, they are only defined within the block they’re declared in; the immediately-invoked function (IIFE) in this case. When we use the typeof operator, the operand x is not defined: we are trying to access x outside of the block it’s declared in. This means that x is not defined. Values who haven’t been assigned a value or declared are of type "undefined"console.log(typeof x) returns "undefined".

However, we created a global variable y when setting y equal to 10. This value is accessible anywhere in our code. y is defined, and holds a value of type "number"console.log(typeof y) returns "number".

Q. What is the output?

class Dog {
  constructor(name) {
    this.name = name;
  }
}

Dog.prototype.bark = function () {
  console.log(`Woof I am ${this.name}`);
};

const pet = new Dog("Mara");

pet.bark();

delete Dog.prototype.bark;

pet.bark();
  • A: "Woof I am Mara"TypeError
  • B: "Woof I am Mara""Woof I am Mara"
  • C: "Woof I am Mara"undefined
  • D: TypeErrorTypeError

Answer: A

We can delete properties from objects using the delete keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the bark function is not available anymore on the prototype after delete Dog.prototype.bark, yet we still try to access it.

When we try to invoke something that is not a function, a TypeError is thrown. In this case TypeError: pet.bark is not a function, since pet.bark is undefined.

Q. What is the output?

const set = new Set([1, 1, 2, 3, 4]);

console.log(set);
  • A: [1, 1, 2, 3, 4]
  • B: [1, 2, 3, 4]
  • C: {1, 1, 2, 3, 4}
  • D: {1, 2, 3, 4}

Answer: D

The Set object is a collection of unique values: a value can only occur once in a set.

We passed the iterable [1, 1, 2, 3, 4] with a duplicate value 1. Since we cannot have two of the same values in a set, one of them is removed. This results in {1, 2, 3, 4}.

Q. What is the output?

// counter.js
let counter = 10;
export default counter;
// index.js
import myCounter from "./counter";

myCounter += 1;

console.log(myCounter);
  • A: 10
  • B: 11
  • C: Error
  • D: NaN

Answer: C

An imported module is read-only: you cannot modify the imported module. Only the module that exports them can change its value.

When we try to increment the value of myCounter, it throws an error: myCounter is read-only and cannot be modified.

Q. What is the output?

const name = "Lydia";
age = 21;

console.log(delete name);
console.log(delete age);
  • A: falsetrue
  • B: "Lydia"21
  • C: truetrue
  • D: undefinedundefined

Answer: A

The delete operator returns a boolean value: true on a successful deletion, else it’ll return false. However, variables declared with the varconst or let keyword cannot be deleted using the delete operator.

The name variable was declared with a const keyword, so its deletion is not successful: false is returned. When we set age equal to 21, we actually added a property called age to the global object. You can successfully delete properties from objects this way, also the global object, so delete age returns true.

Q. What is the output?

const numbers = [1, 2, 3, 4, 5];
const [y] = numbers;

console.log(y);
  • A: [[1, 2, 3, 4, 5]]
  • B: [1, 2, 3, 4, 5]
  • C: 1
  • D: [1]

Answer: C

We can unpack values from arrays or properties from objects through destructuring. For example:

[a, b] = [1, 2];

The value of a is now 1, and the value of b is now 2. What we actually did in the question, is:

[y] = [1, 2, 3, 4, 5];

This means that the value of y is equal to the first value in the array, which is the number 1. When we log y1 is returned.

Q. What is the output?

const user = { name: "Lydia", age: 21 };
const admin = { admin: true, ...user };

console.log(admin);
  • A: { admin: true, user: { name: "Lydia", age: 21 } }
  • B: { admin: true, name: "Lydia", age: 21 }
  • C: { admin: true, user: ["Lydia", 21] }
  • D: { admin: true }

Answer: B

It’s possible to combine objects using the spread operator .... It lets you create copies of the key/value pairs of one object, and add them to another object. In this case, we create copies of the user object, and add them to the admin object. The admin object now contains the copied key/value pairs, which results in { admin: true, name: "Lydia", age: 21 }.

Q. What is the output?

const person = { name: "Lydia" };

Object.defineProperty(person, "age", { value: 21 });

console.log(person);
console.log(Object.keys(person));
  • A: { name: "Lydia", age: 21 }["name", "age"]
  • B: { name: "Lydia", age: 21 }["name"]
  • C: { name: "Lydia"}["name", "age"]
  • D: { name: "Lydia"}["age"]

Answer: B

With the defineProperty method, we can add new properties to an object, or modify existing ones. When we add a property to an object using the defineProperty method, they are by default not enumerable. The Object.keys method returns all enumerable property names from an object, in this case only "name".

Properties added using the defineProperty method are immutable by default. You can override this behavior using the writableconfigurable and enumerable properties. This way, the defineProperty method gives you a lot more control over the properties you’re adding to an object.

Q. What is the output?

const settings = {
  username: "lydiahallie",
  level: 19,
  health: 90,
};

const data = JSON.stringify(settings, ["level", "health"]);
console.log(data);
  • A: "{"level":19, "health":90}"
  • B: "{"username": "lydiahallie"}"
  • C: "["level", "health"]"
  • D: "{"username": "lydiahallie", "level":19, "health":90}"

Answer: A

The second argument of JSON.stringify is the replacer. The replacer can either be a function or an array, and lets you control what and how the values should be stringified.

If the replacer is an array, only the property names included in the array will be added to the JSON string. In this case, only the properties with the names "level" and "health" are included, "username" is excluded. data is now equal to "{"level":19, "health":90}".

If the replacer is a function, this function gets called on every property in the object you’re stringifying. The value returned from this function will be the value of the property when it’s added to the JSON string. If the value is undefined, this property is excluded from the JSON string.

Q. What is the output?

let num = 10;

const increaseNumber = () => num++;
const increasePassedNumber = (number) => number++;

const num1 = increaseNumber();
const num2 = increasePassedNumber(num1);

console.log(num1);
console.log(num2);
  • A: 1010
  • B: 1011
  • C: 1111
  • D: 1112

Answer: A

The unary operator ++ first returns the value of the operand, then increments the value of the operand. The value of num1 is 10, since the increaseNumber function first returns the value of num, which is 10, and only increments the value of num afterwards.

num2 is 10, since we passed num1 to the increasePassedNumbernumber is equal to 10(the value of num1. Again, the unary operator ++ first returns the value of the operand, then increments the value of the operand. The value of number is 10, so num2 is equal to 10.

Q. What is the output?

const value = { number: 10 };

const multiply = (x = { ...value }) => {
  console.log((x.number *= 2));
};

multiply();
multiply();
multiply(value);
multiply(value);
  • A: 204080160
  • B: 20402040
  • C: 20202040
  • D: NaNNaN2040

Answer: C

In ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is "undefined". In this case, we spread the properties of the value object into a new object, so x has the default value of { number: 10 }.

The default argument is evaluated at call time! Every time we call the function, a new object is created. We invoke the multiply function the first two times without passing a value: x has the default value of { number: 10 }. We then log the multiplied value of that number, which is 20.

The third time we invoke multiply, we do pass an argument: the object called value. The *= operator is actually shorthand for x.number = x.number * 2: we modify the value of x.number, and log the multiplied value 20.

The fourth time, we pass the value object again. x.number was previously modified to 20, so x.number *= 2 logs 40.

Q. What is the output?

[1, 2, 3, 4].reduce((x, y) => console.log(x, y));
  • A: 1 2 and 3 3 and 6 4
  • B: 1 2 and 2 3 and 3 4
  • C: 1 undefined and 2 undefined and 3 undefined and 4 undefined
  • D: 1 2 and undefined 3 and undefined 4

Answer: D

The first argument that the reduce method receives is the accumulatorx in this case. The second argument is the current valuey. With the reduce method, we execute a callback function on every element in the array, which could ultimately result in one single value.

In this example, we are not returning any values, we are simply logging the values of the accumulator and the current value.

The value of the accumulator is equal to the previously returned value of the callback function. If you don’t pass the optional initialValue argument to the reduce method, the accumulator is equal to the first element on the first call.

On the first call, the accumulator (x) is 1, and the current value (y) is 2. We don’t return from the callback function, we log the accumulator and current value: 1 and 2 get logged.

If you don’t return a value from a function, it returns undefined. On the next call, the accumulator is undefined, and the current value is 3undefined and 3 get logged.

On the fourth call, we again don’t return from the callback function. The accumulator is again undefined, and the current value is 4undefined and 4 get logged.

Q. With which constructor can we successfully extend the Dog class?

class Dog {
  constructor(name) {
    this.name = name;
  }
}

class Labrador extends Dog {
  // 1
  constructor(name, size) {
    this.size = size;
  }
  // 2
  constructor(name, size) {
    super(name);
    this.size = size;
  }
  // 3
  constructor(size) {
    super(name);
    this.size = size;
  }
  // 4
  constructor(name, size) {
    this.name = name;
    this.size = size;
  }
}
  • A: 1
  • B: 2
  • C: 3
  • D: 4

Answer: B

In a derived class, you cannot access the this keyword before calling super. If you try to do that, it will throw a ReferenceError: 1 and 4 would throw a reference error.

With the super keyword, we call that parent class’s constructor with the given arguments. The parent’s constructor receives the name argument, so we need to pass name to super.

The Labrador class receives two arguments, name since it extends Dog, and size as an extra property on the Labrador class. They both need to be passed to the constructor function on Labrador, which is done correctly using constructor 2.

Q. What is the output?

// index.js
console.log("running index.js");
import { sum } from "./sum.js";
console.log(sum(1, 2));

// sum.js
console.log("running sum.js");
export const sum = (a, b) => a + b;
  • A: running index.jsrunning sum.js3
  • B: running sum.jsrunning index.js3
  • C: running sum.js3running index.js
  • D: running index.jsundefinedrunning sum.js

Answer: B

With the import keyword, all imported modules are pre-parsed. This means that the imported modules get run first, the code in the file which imports the module gets executed after.

This is a difference between require() in CommonJS and import! With require(), you can load dependencies on demand while the code is being run. If we would have used require instead of importrunning index.jsrunning sum.js3 would have been logged to the console.

Q. What is the output?

console.log(Number(2) === Number(2));
console.log(Boolean(false) === Boolean(false));
console.log(Symbol("foo") === Symbol("foo"));
  • A: truetruefalse
  • B: falsetruefalse
  • C: truefalsetrue
  • D: truetruetrue

Answer: A

Every Symbol is entirely unique. The purpose of the argument passed to the Symbol is to give the Symbol a description. The value of the Symbol is not dependent on the passed argument. As we test equality, we are creating two entirely new symbols: the first Symbol('foo'), and the second Symbol('foo'). These two values are unique and not equal to each other, Symbol('foo') === Symbol('foo') returns false.

Q. What is the output?

const name = "Lydia Hallie";
console.log(name.padStart(13));
console.log(name.padStart(2));
  • A: "Lydia Hallie""Lydia Hallie"
  • B: " Lydia Hallie"" Lydia Hallie" ("[13x whitespace]Lydia Hallie""[2x whitespace]Lydia Hallie")
  • C: " Lydia Hallie""Lydia Hallie" ("[1x whitespace]Lydia Hallie""Lydia Hallie")
  • D: "Lydia Hallie""Lyd",

Answer: C

With the padStart method, we can add padding to the beginning of a string. The value passed to this method is the total length of the string together with the padding. The string "Lydia Hallie" has a length of 12name.padStart(13) inserts 1 space at the start of the string, because 12 + 1 is 13.

If the argument passed to the padStart method is smaller than the length of the array, no padding will be added.

Q. What is the output?

console.log("🥑" + "💻");
  • A: "🥑💻"
  • B: 257548
  • C: A string containing their code points
  • D: Error

Answer: A

With the + operator, you can concatenate strings. In this case, we are concatenating the string "🥑" with the string "💻", resulting in "🥑💻".

Q. How can we log the values that are commented out after the console.log statement?

function* startGame() {
  const answer = yield "Do you love JavaScript?";
  if (answer !== "Yes") {
    return "Oh wow... Guess we're gone here";
  }
  return "JavaScript loves you back ❤️";
}

const game = startGame();
console.log(/* 1 */); // Do you love JavaScript?
console.log(/* 2 */); // JavaScript loves you back ❤️
  • A: game.next("Yes").value and game.next().value
  • B: game.next.value("Yes") and game.next.value()
  • C: game.next().value and game.next("Yes").value
  • D: game.next.value() and game.next.value("Yes")

Answer: C

A generator function “pauses” its execution when it sees the yield keyword. First, we have to let the function yield the string “Do you love JavaScript?”, which can be done by calling game.next().value.

Every line is executed, until it finds the first yield keyword. There is a yield keyword on the first line within the function: the execution stops with the first yield! This means that the variable answer is not defined yet!

When we call game.next("Yes").value, the previous yield is replaced with the value of the parameters passed to the next() function, "Yes" in this case. The value of the variable answer is now equal to "Yes". The condition of the if-statement returns false, and JavaScript loves you back ❤️ gets logged.

Q. What is the output?

console.log(String.raw`Hello\nworld`);
  • A: Hello world!
  • B: Hello
         world
  • C: Hello\nworld
  • D: Hello\n
         world

Answer: C

String.raw returns a string where the escapes (\n\v\t etc.) are ignored! Backslashes can be an issue since you could end up with something like:

const path = `C:\Documents\Projects\table.html`

Which would result in:

"C:DocumentsProjects able.html"

With String.raw, it would simply ignore the escape and print:

C:\Documents\Projects\table.html

In this case, the string is Hello\nworld, which gets logged.

Q. What is the output?

async function getData() {
  return await Promise.resolve("I made it!");
}

const data = getData();
console.log(data);
  • A: "I made it!"
  • B: Promise {<resolved>: "I made it!"}
  • C: Promise {<pending>}
  • D: undefined

Answer: C

An async function always returns a promise. The await still has to wait for the promise to resolve: a pending promise gets returned when we call getData() in order to set data equal to it.

If we wanted to get access to the resolved value "I made it", we could have used the .then() method on data:

data.then(res => console.log(res))

This would’ve logged "I made it!"

Q. What is the output?

function addToList(item, list) {
  return list.push(item);
}

const result = addToList("apple", ["banana"]);
console.log(result);
  • A: ['apple', 'banana']
  • B: 2
  • C: true
  • D: undefined

Answer: B

The .push() method returns the length of the new array! Previously, the array contained one element (the string "banana") and had a length of 1. After adding the string "apple" to the array, the array contains two elements, and has a length of 2. This gets returned from the addToList function.

The push method modifies the original array. If you wanted to return the array from the function rather than the length of the array, you should have returned list after pushing item to it.

Q. What is the output?

const box = { x: 10, y: 20 };

Object.freeze(box);

const shape = box;
shape.x = 100;

console.log(shape);
  • A: { x: 100, y: 20 }
  • B: { x: 10, y: 20 }
  • C: { x: 100 }
  • D: ReferenceError

Answer: B

Object.freeze makes it impossible to add, remove, or modify properties of an object (unless the property’s value is another object).

When we create the variable shape and set it equal to the frozen object boxshape also refers to a frozen object. You can check whether an object is frozen by using Object.isFrozen. In this case, Object.isFrozen(shape) returns true, since the variable shape has a reference to a frozen object.

Since shape is frozen, and since the value of x is not an object, we cannot modify the property xx is still equal to 10, and { x: 10, y: 20 } gets logged.

Q. What is the output?

const { name: myName } = { name: "Lydia" };

console.log(name);
  • A: "Lydia"
  • B: "myName"
  • C: undefined
  • D: ReferenceError

Answer: D

When we unpack the property name from the object on the right-hand side, we assign its value "Lydia" to a variable with the name myName.

With { name: myName }, we tell JavaScript that we want to create a new variable called myName with the value of the name property on the right-hand side.

Since we try to log name, a variable that is not defined, a ReferenceError gets thrown.

Q. Is this a pure function?

function sum(a, b) {
  return a + b;
}
  • A: Yes
  • B: No

Answer: A

A pure function is a function that always returns the same result, if the same arguments are passed.

The sum function always returns the same result. If we pass 1 and 2, it will always return 3 without side effects. If we pass 5 and 10, it will always return 15, and so on. This is the definition of a pure function.

Q. What is the output?

const add = () => {
  const cache = {};
  return (num) => {
    if (num in cache) {
      return `From cache! ${cache[num]}`;
    } else {
      const result = num + 10;
      cache[num] = result;
      return `Calculated! ${result}`;
    }
  };
};

const addFunction = add();
console.log(addFunction(10));
console.log(addFunction(10));
console.log(addFunction(5 * 2));
  • A: Calculated! 20 Calculated! 20 Calculated! 20
  • B: Calculated! 20 From cache! 20 Calculated! 20
  • C: Calculated! 20 From cache! 20 From cache! 20
  • D: Calculated! 20 From cache! 20 Error

Answer: C

The add function is a memoized function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a cache object that stores the previously returned values.

If we call the addFunction function again with the same argument, it first checks whether it has already gotten that value in its cache. If that’s the case, the caches value will be returned, which saves on execution time. Else, if it’s not cached, it will calculate the value and store it afterwards.

We call the addFunction function three times with the same value: on the first invocation, the value of the function when num is equal to 10 isn’t cached yet. The condition of the if-statement num in cache returns false, and the else block gets executed: Calculated! 20 gets logged, and the value of the result gets added to the cache object. cache now looks like { 10: 20 }.

The second time, the cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.

The third time, we pass 5 * 2 to the function which gets evaluated to 10. The cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.

Q. What is the output?

const myLifeSummedUp = ["☕", "💻", "🍷", "🍫"];

for (let item in myLifeSummedUp) {
  console.log(item);
}

for (let item of myLifeSummedUp) {
  console.log(item);
}
  • A: 0 1 2 3 and "☕" "💻" "🍷" "🍫"
  • B: "☕" "💻" "🍷" "🍫" and "☕" "💻" "🍷" "🍫"
  • C: "☕" "💻" "🍷" "🍫" and 0 1 2 3
  • D: 0 1 2 3 and {0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}

Answer: A

With a for-in loop, we can iterate over enumerable properties. In an array, the enumerable properties are the “keys” of array elements, which are actually their indexes. You could see an array as:

{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}

Where the keys are the enumerable properties. 0 1 2 3 get logged.

With a for-of loop, we can iterate over iterables. An array is an iterable. When we iterate over the array, the variable “item” is equal to the element it’s currently iterating over, "☕" "💻" "🍷" "🍫" get logged.

Q. What is the output?

const list = [1 + 2, 1 * 2, 1 / 2];
console.log(list);
  • A: ["1 + 2", "1 * 2", "1 / 2"]
  • B: ["12", 2, 0.5]
  • C: [3, 2, 0.5]
  • D: [1, 1, 1]

Answer: C

Array elements can hold any value. Numbers, strings, objects, other arrays, null, boolean values, undefined, and other expressions such as dates, functions, and calculations.

The element will be equal to the returned value. 1 + 2 returns 31 * 2 returns 2, and 1 / 2 returns 0.5.

Q. What is the output?

function sayHi(name) {
  return `Hi there, ${name}`;
}

console.log(sayHi());
  • A: Hi there,
  • B: Hi there, undefined
  • C: Hi there, null
  • D: ReferenceError

Answer: B

By default, arguments have the value of undefined, unless a value has been passed to the function. In this case, we didn’t pass a value for the name argument. name is equal to undefined which gets logged.

In ES6, we can overwrite this default undefined value with default parameters. For example:

function sayHi(name = "Lydia") { ... }

In this case, if we didn’t pass a value or if we passed undefinedname would always be equal to the string Lydia

Q. What is the output?

var status = "😎";

setTimeout(() => {
  const status = "😍";

  const data = {
    status: "🥑",
    getStatus() {
      return this.status;
    },
  };

  console.log(data.getStatus());
  console.log(data.getStatus.call(this));
}, 0);
  • A: "🥑" and "😍"
  • B: "🥑" and "😎"
  • C: "😍" and "😎"
  • D: "😎" and "😎"

Answer: B

The value of the this keyword is dependent on where you use it. In a method, like the getStatus method, the this keyword refers to the object that the method belongs to. The method belongs to the data object, so this refers to the data object. When we log this.status, the status property on the data object gets logged, which is "🥑".

With the call method, we can change the object to which the this keyword refers. In functions, the this keyword refers to the the object that the function belongs to. We declared the setTimeout function on the global object, so within the setTimeout function, the this keyword refers to the global object. On the global object, there is a variable called status with the value of "😎". When logging this.status"😎" gets logged.

Q. What is the output?

const person = {
  name: "Lydia",
  age: 21,
};

let city = person.city;
city = "Amsterdam";

console.log(person);
  • A: { name: "Lydia", age: 21 }
  • B: { name: "Lydia", age: 21, city: "Amsterdam" }
  • C: { name: "Lydia", age: 21, city: undefined }
  • D: "Amsterdam"

Answer: A

We set the variable city equal to the value of the property called city on the person object. There is no property on this object called city, so the variable city has the value of undefined.

Note that we are not referencing the person object itself! We simply set the variable city equal to the current value of the city property on the person object.

Then, we set city equal to the string "Amsterdam". This doesn’t change the person object: there is no reference to that object.

When logging the person object, the unmodified object gets returned.

Q. What is the output?

function checkAge(age) {
  if (age < 18) {
    const message = "Sorry, you're too young.";
  } else {
    const message = "Yay! You're old enough!";
  }

  return message;
}

console.log(checkAge(21));
  • A: "Sorry, you're too young."
  • B: "Yay! You're old enough!"
  • C: ReferenceError
  • D: undefined

Answer: C

Variables with the const and let keyword are block-scoped. A block is anything between curly brackets ({ }). In this case, the curly brackets of the if/else statements. You cannot reference a variable outside of the block it’s declared in, a ReferenceError gets thrown.

Q. What kind of information would get logged?

fetch("https://www.website.com/api/user/1")
  .then((res) => res.json())
  .then((res) => console.log(res));
  • A: The result of the fetch method.
  • B: The result of the second invocation of the fetch method.
  • C: The result of the callback in the previous .then().
  • D: It would always be undefined.

Answer: C

The value of res in the second .then is equal to the returned value of the previous .then. You can keep chaining .thens like this, where the value is passed to the next handler.

Q. Which option is a way to set hasName equal to true, provided you cannot pass true as an argument?

function getName(name) {
  const hasName = //
}
  • A: !!name
  • B: name
  • C: new Boolean(name)
  • D: name.length

Answer: A

With !!name, we determine whether the value of name is truthy or falsy. If name is truthy, which we want to test for, !name returns false!false (which is what !!name practically is) returns true.

By setting hasName equal to name, you set hasName equal to whatever value you passed to the getName function, not the boolean value true.

new Boolean(true) returns an object wrapper, not the boolean value itself.

name.length returns the length of the passed argument, not whether it’s true.

Q. What is the output?

console.log("I want pizza"[0]);
  • A: """
  • B: "I"
  • C: SyntaxError
  • D: undefined

Answer: B

In order to get an character on a specific index in a string, you can use bracket notation. The first character in the string has index 0, and so on. In this case we want to get the element which index is 0, the character "I', which gets logged.

Note that this method is not supported in IE7 and below. In that case, use .charAt()

Q. What is the output?

function sum(num1, num2 = num1) {
  console.log(num1 + num2);
}

sum(10);
  • A: NaN
  • B: 20
  • C: ReferenceError
  • D: undefined

Answer: B

You can set a default parameter’s value equal to another parameter of the function, as long as they’ve been defined before the default parameter. We pass the value 10 to the sum function. If the sum function only receives 1 argument, it means that the value for num2 is not passed, and the value of num1 is equal to the passed value 10 in this case. The default value of num2 is the value of num1, which is 10num1 + num2 returns 20.

If you’re trying to set a default parameter’s value equal to a parameter which is defined after (to the right), the parameter’s value hasn’t been initialized yet, which will throw an error.

Q. What is the output?

// module.js
export default () => "Hello world";
export const name = "Lydia";

// index.js
import * as data from "./module";

console.log(data);
  • A: { default: function default(), name: "Lydia" }
  • B: { default: function default() }
  • C: { default: "Hello world", name: "Lydia" }
  • D: Global object of module.js

Answer: A

With the import * as name syntax, we import all exports from the module.js file into the index.js file as a new object called data is created. In the module.js file, there are two exports: the default export, and a named export. The default export is a function which returns the string "Hello World", and the named export is a variable called name which has the value of the string "Lydia".

The data object has a default property for the default export, other properties have the names of the named exports and their corresponding values.

Q. What is the output?

class Person {
  constructor(name) {
    this.name = name;
  }
}

const member = new Person("John");
console.log(typeof member);
  • A: "class"
  • B: "function"
  • C: "object"
  • D: "string"

Answer: C

Classes are syntactical sugar for function constructors. The equivalent of the Person class as a function constructor would be:

function Person() {
  this.name = name;
}

Calling a function constructor with new results in the creation of an instance of Persontypeof keyword returns "object" for an instance. typeof member returns "object".

Q. What is the output?

let newList = [1, 2, 3].push(4);

console.log(newList.push(5));
  • A: [1, 2, 3, 4, 5]
  • B: [1, 2, 3, 5]
  • C: [1, 2, 3, 4]
  • D: Error

Answer: D

The .push method returns the new length of the array, not the array itself! By setting newList equal to [1, 2, 3].push(4), we set newList equal to the new length of the array: 4.

Then, we try to use the .push method on newList. Since newList is the numerical value 4, we cannot use the .push method: a TypeError is thrown.

Q. What is the output?

function giveLydiaPizza() {
  return "Here is pizza!";
}

const giveLydiaChocolate = () =>
  "Here's chocolate... now go hit the gym already.";

console.log(giveLydiaPizza.prototype);
console.log(giveLydiaChocolate.prototype);
  • A: { constructor: ...} { constructor: ...}
  • B: {} { constructor: ...}
  • C: { constructor: ...} {}
  • D: { constructor: ...} undefined

Answer: D

Regular functions, such as the giveLydiaPizza function, have a prototype property, which is an object (prototype object) with a constructor property. Arrow functions however, such as the giveLydiaChocolate function, do not have this prototype property. undefined gets returned when trying to access the prototype property using giveLydiaChocolate.prototype.

Q. What is the output?

const person = {
  name: "Lydia",
  age: 21,
};

for (const [x, y] of Object.entries(person)) {
  console.log(x, y);
}
  • A: name Lydia and age 21
  • B: ["name", "Lydia"] and ["age", 21]
  • C: ["name", "age"] and undefined
  • D: Error

Answer: A

Object.entries(person) returns an array of nested arrays, containing the keys and objects:

[ [ 'name', 'Lydia' ], [ 'age', 21 ] ]

Using the for-of loop, we can iterate over each element in the array, the subarrays in this case. We can destructure the subarrays instantly in the for-of loop, using const [x, y]x is equal to the first element in the subarray, y is equal to the second element in the subarray.

The first subarray is [ "name", "Lydia" ], with x equal to "name", and y equal to "Lydia", which get logged. The second subarray is [ "age", 21 ], with x equal to "age", and y equal to 21, which get logged.

Q. What is the output?

function getItems(fruitList, ...args, favoriteFruit) {
  return [...fruitList, ...args, favoriteFruit]
}

getItems(["banana", "apple"], "pear", "orange")
  • A: ["banana", "apple", "pear", "orange"]
  • B: [["banana", "apple"], "pear", "orange"]
  • C: ["banana", "apple", ["pear"], "orange"]
  • D: SyntaxError

Answer: D

...args is a rest parameter. The rest parameter’s value is an array containing all remaining arguments, and can only be the last parameter! In this example, the rest parameter was the second parameter. This is not possible, and will throw a syntax error.

function getItems(fruitList, favoriteFruit, ...args) {
  return [...fruitList, ...args, favoriteFruit];
}

getItems(["banana", "apple"], "pear", "orange");

The above example works. This returns the array [ 'banana', 'apple', 'orange', 'pear' ]

Q. What is the output?

function nums(a, b) {
  if (a > b) console.log("a is bigger");
  else console.log("b is bigger");
  return;
  a + b;
}

console.log(nums(4, 2));
console.log(nums(1, 2));
  • A: a is bigger6 and b is bigger3
  • B: a is biggerundefined and b is biggerundefined
  • C: undefined and undefined
  • D: SyntaxError

Answer: B

In JavaScript, we don’t have to write the semicolon (;) explicitly, however the JavaScript engine still adds them after statements. This is called Automatic Semicolon Insertion. A statement can for example be variables, or keywords like throwreturnbreak, etc.

Here, we wrote a return statement, and another value a + b on a new line. However, since it’s a new line, the engine doesn’t know that it’s actually the value that we wanted to return. Instead, it automatically added a semicolon after return. You could see this as:

return;
a + b;

This means that a + b is never reached, since a function stops running after the return keyword. If no value gets returned, like here, the function returns undefined. Note that there is no automatic insertion after if/else statements!

Q. What is the output?

class Person {
  constructor() {
    this.name = "Lydia";
  }
}

Person = class AnotherPerson {
  constructor() {
    this.name = "Sarah";
  }
};

const member = new Person();
console.log(member.name);
  • A: "Lydia"
  • B: "Sarah"
  • C: Error: cannot redeclare Person
  • D: SyntaxError

Answer: B

We can set classes equal to other classes/function constructors. In this case, we set Person equal to AnotherPerson. The name on this constructor is Sarah, so the name property on the new Person instance member is "Sarah".

Q. What is the output?

const info = {
  [Symbol("a")]: "b",
};

console.log(info);
console.log(Object.keys(info));
  • A: {Symbol('a'): 'b'} and ["{Symbol('a')"]
  • B: {} and []
  • C: { a: "b" } and ["a"]
  • D: {Symbol('a'): 'b'} and []

Answer: D

A Symbol is not enumerable. The Object.keys method returns all enumerable key properties on an object. The Symbol won’t be visible, and an empty array is returned. When logging the entire object, all properties will be visible, even non-enumerable ones.

This is one of the many qualities of a symbol: besides representing an entirely unique value (which prevents accidental name collision on objects, for example when working with 2 libraries that want to add properties to the same object), you can also “hide” properties on objects this way (although not entirely. You can still access symbols using the Object.getOwnPropertySymbols() method).

Q. What is the output?

const getList = ([x, ...y]) => [x, y]
const getUser = user => { name: user.name, age: user.age }

const list = [1, 2, 3, 4]
const user = { name: "Lydia", age: 21 }

console.log(getList(list))
console.log(getUser(user))
  • A: [1, [2, 3, 4]] and undefined
  • B: [1, [2, 3, 4]] and { name: "Lydia", age: 21 }
  • C: [1, 2, 3, 4] and { name: "Lydia", age: 21 }
  • D: Error and { name: "Lydia", age: 21 }

Answer: A

The getList function receives an array as its argument. Between the parentheses of the getList function, we destructure this array right away. You could see this as:

[x, ...y] = [1, 2, 3, 4]

With the rest parameter ...y, we put all “remaining” arguments in an array. The remaining arguments are 23 and 4 in this case. The value of y is an array, containing all the rest parameters. The value of x is equal to 1 in this case, so when we log [x, y][1, [2, 3, 4]] gets logged.

The getUser function receives an object. With arrow functions, we don’t have to write curly brackets if we just return one value. However, if you want to return an object from an arrow function, you have to write it between parentheses, otherwise no value gets returned! The following function would have returned an object:

const getUser = user => ({ name: user.name, age: user.age })

Since no value gets returned in this case, the function returns undefined.

Q. What is the output?

const name = "Lydia";

console.log(name());
  • A: SyntaxError
  • B: ReferenceError
  • C: TypeError
  • D: undefined

Answer: C

The variable name holds the value of a string, which is not a function, thus cannot invoke.

TypeErrors get thrown when a value is not of the expected type. JavaScript expected name to be a function since we’re trying to invoke it. It was a string however, so a TypeError gets thrown: name is not a function!

SyntaxErrors get thrown when you’ve written something that isn’t valid JavaScript, for example when you’ve written the word return as retrun. ReferenceErrors get thrown when JavaScript isn’t able to find a reference to a value that you’re trying to access.

Q. What is the value of output?

// 🎉✨ This is my 100th question! ✨🎉

const output = `${[] && "Im"}possible!
You should${"" && `n't`} see a therapist after so much JavaScript lol`;
  • A: possible! You should see a therapist after so much JavaScript lol
  • B: Impossible! You should see a therapist after so much JavaScript lol
  • C: possible! You shouldn't see a therapist after so much JavaScript lol
  • D: Impossible! You shouldn't see a therapist after so much JavaScript lol

Answer: B

[] is a truthy value. With the && operator, the right-hand value will be returned if the left-hand value is a truthy value. In this case, the left-hand value [] is a truthy value, so "Im' gets returned.

"" is a falsy value. If the left-hand value is falsy, nothing gets returned. n't doesn’t get returned.

Q. What is the value of output?

const one = false || {} || null;
const two = null || false || "";
const three = [] || 0 || true;

console.log(one, two, three);
  • A: false null []
  • B: null "" true
  • C: {} "" []
  • D: null null true

Answer: C

With the || operator, we can return the first truthy operand. If all values are falsy, the last operand gets returned.

(false || {} || null): the empty object {} is a truthy value. This is the first (and only) truthy value, which gets returned. one is equal to {}.

(null || false || ""): all operands are falsy values. This means that the past operand, "" gets returned. two is equal to "".

([] || 0 || ""): the empty array[] is a truthy value. This is the first truthy value, which gets returned. three is equal to [].

Q. What is the value of output?

const myPromise = () => Promise.resolve("I have resolved!");

function firstFunction() {
  myPromise().then((res) => console.log(res));
  console.log("second");
}

async function secondFunction() {
  console.log(await myPromise());
  console.log("second");
}

firstFunction();
secondFunction();
  • A: I have resolved!second and I have resolved!second
  • B: secondI have resolved! and secondI have resolved!
  • C: I have resolved!second and secondI have resolved!
  • D: secondI have resolved! and I have resolved!second

Answer: D

With a promise, we basically say I want to execute this function, but I’ll put it aside for now while it’s running since this might take a while. Only when a certain value is resolved (or rejected), and when the call stack is empty, I want to use this value.

We can get this value with both .then and the await keyword in an async function. Although we can get a promise’s value with both .then and await, they work a bit differently.

In the firstFunction, we (sort of) put the myPromise function aside while it was running, but continued running the other code, which is console.log('second') in this case. Then, the function resolved with the string I have resolved, which then got logged after it saw that the callstack was empty.

With the await keyword in secondFunction, we literally pause the execution of an async function until the value has been resolved befoer moving to the next line.

This means that it waited for the myPromise to resolve with the value I have resolved, and only once that happened, we moved to the next line: second got logged.

Q. What is the value of output?

const set = new Set();

set.add(1);
set.add("Lydia");
set.add({ name: "Lydia" });

for (let item of set) {
  console.log(item + 2);
}
  • A: 3NaNNaN
  • B: 37NaN
  • C: 3Lydia2[Object object]2
  • D: "12"Lydia2[Object object]2

Answer: C

The + operator is not only used for adding numerical values, but we can also use it to concatenate strings. Whenever the JavaScript engine sees that one or more values are not a number, it coerces the number into a string.

The first one is 1, which is a numerical value. 1 + 2 returns the number 3.

However, the second one is a string "Lydia""Lydia" is a string and 2 is a number: 2 gets coerced into a string. "Lydia" and "2" get concatenated, which results in the string "Lydia2".

{ name: "Lydia" } is an object. Neither a number nor an object is a string, so it stringifies both. Whenever we stringify a regular object, it becomes "[Object object]""[Object object]" concatenated with "2" becomes "[Object object]2".

Q. What is its value?

Promise.resolve(5);
  • A: 5
  • B: Promise {<pending>: 5}
  • C: Promise {<resolved>: 5}
  • D: Error

Answer: C

We can pass any type of value we want to Promise.resolve, either a promise or a non-promise. The method itself returns a promise with the resolved value. If you pass a regular function, it’ll be a resolved promise with a regular value. If you pass a promise, it’ll be a resolved promise with the resolved value of that passed promise.

In this case, we just passed the numerical value 5. It returns a resolved promise with the value 5.

Q. What is its value?

function compareMembers(person1, person2 = person) {
  if (person1 !== person2) {
    console.log("Not the same!");
  } else {
    console.log("They are the same!");
  }
}

const person = { name: "Lydia" };

compareMembers(person);
  • A: Not the same!
  • B: They are the same!
  • C: ReferenceError
  • D: SyntaxError

Answer: B

Objects are passed by reference. When we check objects for strict equality (===), we’re comparing their references.

We set the default value for person2 equal to the person object, and passed the person object as the value for person1.

This means that both values have a reference to the same spot in memory, thus they are equal.

The code block in the else statement gets run, and They are the same! gets logged.

Q. What is its value?

const colorConfig = {
  red: true,
  blue: false,
  green: true,
  black: true,
  yellow: false,
};

const colors = ["pink", "red", "blue"];

console.log(colorConfig.colors[1]);
  • A: true
  • B: false
  • C: undefined
  • D: TypeError

Answer: D

In JavaScript, we have two ways to access properties on an object: bracket notation, or dot notation. In this example, we use dot notation (colorConfig.colors) instead of bracket notation (colorConfig["colors"]).

With dot notation, JavaScript tries to find the property on the object with that exact name. In this example, JavaScript tries to find a property called colors on the colorConfig object. There is no proprety called colors, so this returns undefined. Then, we try to access the value of the first element by using [1]. We cannot do this on a value that’s undefined, so it throws a TypeErrorCannot read property '1' of undefined.

JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [ and keeps going until it finds the closing bracket ]. Only then, it will evaluate the statement. If we would’ve used colorConfig[colors[1]], it would have returned the value of the red property on the colorConfig object.

Q. What is its value?

console.log("❤️" === "❤️");
  • A: true
  • B: false

Answer: A

Under the hood, emojis are unicodes. The unicodes for the heart emoji is "U+2764 U+FE0F". These are always the same for the same emojis, so we’re comparing two equal strings to each other, which returns true.

Q. Which of these methods modifies the original array?

const emojis = ["✨", "🥑", "😍"];

emojis.map((x) => x + "✨");
emojis.filter((x) => x !== "🥑");
emojis.find((x) => x !== "🥑");
emojis.reduce((acc, cur) => acc + "✨");
emojis.slice(1, 2, "✨");
emojis.splice(1, 2, "✨");
  • A: All of them
  • B: map reduce slice splice
  • C: map slice splice
  • D: splice

Answer: D

With splice method, we modify the original array by deleting, replacing or adding elements. In this case, we removed 2 items from index 1 (we removed '🥑' and '😍') and added the ✨ emoji instead.

mapfilter and slice return a new array, find returns an element, and reduce returns a reduced value.

Q. What is the output?

const food = ["🍕", "🍫", "🥑", "🍔"];
const info = { favoriteFood: food[0] };

info.favoriteFood = "🍝";

console.log(food);
  • A: ['🍕', '🍫', '🥑', '🍔']
  • B: ['🍝', '🍫', '🥑', '🍔']
  • C: ['🍝', '🍕', '🍫', '🥑', '🍔']
  • D: ReferenceError

Answer: A

We set the value of the favoriteFood property on the info object equal to the string with the pizza emoji, '🍕'. A string is a primitive data type. In JavaScript, primitive data types act by reference

In JavaScript, primitive data types (everything that’s not an object) interact by value. In this case, we set the value of the favoriteFood property on the info object equal to the value of the first element in the food array, the string with the pizza emoji in this case ('🍕'). A string is a primitive data type, and interact by value (see my blogpost if you’re interested in learning more)

Then, we change the value of the favoriteFood property on the info object. The food array hasn’t changed, since the value of favoriteFood was merely a copy of the value of the first element in the array, and doesn’t have a reference to the same spot in memory as the element on food[0]. When we log food, it’s still the original array, ['🍕', '🍫', '🥑', '🍔'].

Q. What does this method do?

JSON.parse();
  • A: Parses JSON to a JavaScript value
  • B: Parses a JavaScript object to JSON
  • C: Parses any JavaScript value to JSON
  • D: Parses JSON to a JavaScript object only

Answer: A

With the JSON.parse() method, we can parse JSON string to a JavaScript value.

// Stringifying a number into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonNumber = JSON.stringify(4); // '4'
JSON.parse(jsonNumber); // 4

// Stringifying an array value into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonArray = JSON.stringify([1, 2, 3]); // '[1, 2, 3]'
JSON.parse(jsonArray); // [1, 2, 3]

// Stringifying an object  into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonArray = JSON.stringify({ name: "Lydia" }); // '{"name":"Lydia"}'
JSON.parse(jsonArray); // { name: 'Lydia' }

Q. What is the output?

let name = "Lydia";

function getName() {
  console.log(name);
  let name = "Sarah";
}

getName();
  • A: Lydia
  • B: Sarah
  • C: undefined
  • D: ReferenceError

Answer: D

Each function has its own execution context (or scope). The getName function first looks within its own context (scope) to see if it contains the variable name we’re trying to access. In this case, the getName function contains its own name variable: we declare the variable name with the let keyword, and with the value of 'Sarah'.

Variables with the let keyword (and const) are hoisted, but unlike var, don’t get initialized. They are not accessible before the line we declare (initialize) them. This is called the “temporal dead zone”. When we try to access the variables before they are declared, JavaScript throws a ReferenceError.

If we wouldn’t have declared the name variable within the getName function, the javascript engine would’ve looked down the scope chain. The outer scope has a variable called name with the value of Lydia. In that case, it would’ve logged Lydia.

let name = "Lydia";

function getName() {
  console.log(name);
}

getName(); // Lydia

Q. What is the output?

function* generatorOne() {
  yield ["a", "b", "c"];
}

function* generatorTwo() {
  yield* ["a", "b", "c"];
}

const one = generatorOne();
const two = generatorTwo();

console.log(one.next().value);
console.log(two.next().value);
  • A: a and a
  • B: a and undefined
  • C: ['a', 'b', 'c'] and a
  • D: a and ['a', 'b', 'c']

Answer: C

With the yield keyword, we yield values in a generator function. With the yield* keyword, we can yield values from another generator function, or iterable object (for example an array).

In generatorOne, we yield the entire array ['a', 'b', 'c'] using the yield keyword. The value of value property on the object returned by the next method on one (one.next().value) is equal to the entire array ['a', 'b', 'c'].

console.log(one.next().value); // ['a', 'b', 'c']
console.log(one.next().value); // undefined

In generatorTwo, we use the yield* keyword. This means that the first yielded value of two, is equal to the first yielded value in the iterator. The iterator is the array ['a', 'b', 'c']. The first yielded value is a, so the first time we call two.next().valuea is returned.

console.log(two.next().value); // 'a'
console.log(two.next().value); // 'b'
console.log(two.next().value); // 'c'
console.log(two.next().value); // undefined

Q. What is the output?

console.log(`${((x) => x)("I love")} to program`);
  • A: I love to program
  • B: undefined to program
  • C: ${(x => x)('I love') to program
  • D: TypeError

Answer: A

Expressions within template literals are evaluated first. This means that the string will contain the returned value of the expression, the immediately invoked function (x => x)('I love') in this case. We pass the value 'I love' as an argument to the x => x arrow function. x is equal to 'I love', which gets returned. This results in I love to program.

Q. What will happen?

let config = {
  alert: setInterval(() => {
    console.log("Alert!");
  }, 1000),
};

config = null;
  • A: The setInterval callback won’t be invoked
  • B: The setInterval callback gets invoked once
  • C: The setInterval callback will still be called every second
  • D: We never invoked config.alert(), config is null

Answer: C

Normally when we set objects equal to null, those objects get garbage collected as there is no reference anymore to that object. However, since the callback function within setInterval is an arrow function (thus bound to the config object), the callback function still holds a reference to the config object. As long as there is a reference, the object won’t get garbage collected. Since it’s not garbage collected, the setInterval callback function will still get invoked every 1000ms (1s).

Q. Which method(s) will return the value 'Hello world!'?

const myMap = new Map();
const myFunc = () => "greeting";

myMap.set(myFunc, "Hello world!");

//1
myMap.get("greeting");
//2
myMap.get(myFunc);
//3
myMap.get(() => "greeting");
  • A: 1
  • B: 2
  • C: 2 and 3
  • D: All of them

Answer: B

When adding a key/value pair using the set method, the key will be the value of the first argument passed to the set function, and the value will be the second argument passed to the set function. The key is the function () => 'greeting' in this case, and the value 'Hello world'myMap is now { () => 'greeting' => 'Hello world!' }.

1 is wrong, since the key is not 'greeting' but () => 'greeting'. 3 is wrong, since we’re creating a new function by passing it as a parameter to the get method. Object interact by reference. Functions are objects, which is why two functions are never strictly equal, even if they are identical: they have a reference to a different spot in memory.

Q. What is the output?

const person = {
  name: "Lydia",
  age: 21,
};

const changeAge = (x = { ...person }) => (x.age += 1);
const changeAgeAndName = (x = { ...person }) => {
  x.age += 1;
  x.name = "Sarah";
};

changeAge(person);
changeAgeAndName();

console.log(person);
  • A: {name: "Sarah", age: 22}
  • B: {name: "Sarah", age: 23}
  • C: {name: "Lydia", age: 22}
  • D: {name: "Lydia", age: 23}

Answer: C

Both the changeAge and changeAgeAndName functions have a default parameter, namely a newly created object { ...person }. This object has copies of all the key/values in the person object.

First, we invoke the changeAge function and pass the person object as its argument. This function increases the value of the age property by 1. person is now { name: "Lydia", age: 22 }.

Then, we invoke the changeAgeAndName function, however we don’t pass a parameter. Instead, the value of x is equal to a new object: { ...person }. Since it’s a new object, it doesn’t affect the values of the properties on the person object. person is still equal to { name: "Lydia", age: 22 }.

Q. Predict the output

if(2 == true) // returns false

if(2 == false) // returns false

Q. Being told that an unsorted array contains (n – 1) of n consecutive numbers (where the bounds are defined), find the missing number in O(n) time?

// The output of the function should be 8
var arrayOfIntegers = [2, 5, 1, 4, 9, 6, 3, 7];
var upperBound = 9;
var lowerBound = 1;

findMissingNumber(arrayOfIntegers, upperBound, lowerBound); // 8

function findMissingNumber(arrayOfIntegers, upperBound, lowerBound) {
  // Iterate through array to find the sum of the numbers
  var sumOfIntegers = 0;
  for (var i = 0; i < arrayOfIntegers.length; i++) {
    sumOfIntegers += arrayOfIntegers[i];
  }

  // Find theoretical sum of the consecutive numbers using a variation of Gauss Sum.
  // Formula: [(N * (N + 1)) / 2] - [(M * (M - 1)) / 2];
  // N is the upper bound and M is the lower bound

  upperLimitSum = (upperBound * (upperBound + 1)) / 2;
  lowerLimitSum = (lowerBound * (lowerBound - 1)) / 2;

  theoreticalSum = upperLimitSum - lowerLimitSum;

  return theoreticalSum - sumOfIntegers;
}

// Output
// 8 

Q. How will you remove duplicates from an array in JavaScript?

a.) Using set()

const names = ['John', 'Paul', 'George', 'Ringo', 'John'];

let unique = [...new Set(names)];
console.log(unique); // 'John', 'Paul', 'George', 'Ringo'

b.) Using filter()

const names = ['John', 'Paul', 'George', 'Ringo', 'John'];

let x = (names) => names.filter((v,i) => names.indexOf(v) === i)
x(names); // 'John', 'Paul', 'George', 'Ringo'

c.) Using forEach()

const names = ['John', 'Paul', 'George', 'Ringo', 'John'];

function removeDups(names) {
  let unique = {};
  names.forEach(function(i) {
    if(!unique[i]) {
      unique[i] = true;
    }
  });
  return Object.keys(unique);
}

removeDups(names); // // 'John', 'Paul', 'George', 'Ringo'

d.) Using set()

// ES6 Implementation
var array = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8];

Array.from(new Set(array)); // [1, 2, 3, 5, 9, 8]

e.) Using Hashmap

// ES5 Implementation
var array = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8];

uniqueArray(array); // [1, 2, 3, 5, 9, 8]

function uniqueArray(array) {
  var hashmap = {};
  var unique = [];

  for(var i = 0; i < array.length; i++) {
    // If key returns undefined (unique), it is evaluated as false.
    if(!hashmap.hasOwnProperty(array[i])) {
      hashmap[array[i]] = 1;
      unique.push(array[i]);
    }
  }

  return unique;
}

Q. Given a string, reverse each word in the sentence

var string = "Welcome to this Javascript Guide!";

// Output becomes !ediuG tpircsavaJ siht ot emocleW

var reverseEntireSentence = reverseBySeparator(string, "");

// Output becomes emocleW ot siht tpircsavaJ !ediuG
var reverseEachWord = reverseBySeparator(reverseEntireSentence, " ");

function reverseBySeparator(string, separator) {
  return string.split(separator).reverse().join(separator);
}

Q. Implement enqueue and dequeue using only two stacks

Enqueue means to add an element, dequeue to remove an element.

var inputStack = []; // First stack
var outputStack = []; // Second stack

// For enqueue, just push the item into the first stack
function enqueue(stackInput, item) {
  return stackInput.push(item);
}

function dequeue(stackInput, stackOutput) {
  // Reverse the stack such that the first element of the output stack is the
  // last element of the input stack. After that, pop the top of the output to
  // get the first element that was ever pushed into the input stack
  if (stackOutput.length <= 0) {
    while(stackInput.length > 0) {
      var elementToOutput = stackInput.pop();
      stackOutput.push(elementToOutput);
    }
  }

  return stackOutput.pop();
}

Q. Implement enqueue and dequeue using only two stacks

Enqueue means to add an element, dequeue to remove an element.

var inputStack = []; // First stack
var outputStack = []; // Second stack

// For enqueue, just push the item into the first stack
function enqueue(stackInput, item) {
  return stackInput.push(item);
}

function dequeue(stackInput, stackOutput) {
  // Reverse the stack such that the first element of the output stack is the
  // last element of the input stack. After that, pop the top of the output to
  // get the first element that was ever pushed into the input stack
  if (stackOutput.length <= 0) {
    while(stackInput.length > 0) {
      var elementToOutput = stackInput.pop();
      stackOutput.push(elementToOutput);
    }
  }

  return stackOutput.pop();
}

Q. How would you use a closure to create a private counter?

You can create a function within an outer function (a closure) that allows you to update a private variable but the variable wouldn’t be accessible from outside the function without the use of a helper function.

function counter() {
  var _counter = 0;
  // return an object with several functions that allow you
  // to modify the private _counter variable
  return {
    add: function(increment) { _counter += increment; },
    retrieve: function() { return 'The counter is currently at: ' + _counter; }
  }
}

// error if we try to access the private variable like below
// _counter;

// usage of our counter function
var c = counter();
c.add(5); 
c.add(9); 

// now we can access the private variable in the following way
c.retrieve(); // => The counter is currently at: 14