Javascript Interview Questions

This repo contains list of javascript interview questions

Variables and DataTypes

What is the difference between primitive and reference data type?

What is the difference between let var and const in JavaScript?

What's the difference between null undefined and not defined in JavaScript?

How to check the type of value?

What is this keyword?

What is NaN in javascript?

What is the output of the following:

console.log(2 + 3);
console.log(2 + '3');

console.log(2 + 3 + 4);
console.log(2 + '3' + 4);
console.log(2 + '3' + '4');

console.log(false + true);
console.log(true + true);
console.log(false + false);

Hoisting

What is hoisting?

What is the output of the following:

console.log(x);
console.log(y);
console.log(z);

var x = 10;
let y = 10;
const z = 10;

What is the output of the following:

f1();
f2();

function f1() {
  console.log(1);
}

var f2 = function() {
  console.log(2);
};

Functions

What are the different ways to define function in Javascript?

What is the output of the following:

  function f1() {
    return
    {
      a: 1,
      b: 2
    }
  }
  console.log(f1().a);

What is the difference between function, method and constructor?

What is the difference between normal function and arrow function?

What is the output of the following:

  function f1() {
    console.log(this);
  }
  
  var f2 = () => {
    console.log(this);
  }
  
  f1();
  f2();
  
  var obj = {
    f1: function() {
      console.log(this);
    },
    f2: function() {
      setTimeout(function() {
        console.log(this);
      }, 0);
    },
    f3: function() {
      setTimeout(() => {
        console.log(this);
      }, 0);
    },
  };
  obj.f1();
  obj.f2();
  obj.f3();

What is the difference between call, apply and bind method?

What is arguments keyword?

What is IIFE (Immediately Invoked Function Expression)?

What is callback?

What is the difference between setTimeout and setInterval?

What is closure and what are its advantages and disadvantages?

Define add function which can be invoked like below:

add(1)(2)(3); // return 6

What is the difference between debouncing and throttling?

Equality Operators

What is the difference between == and ===?

What is the output of the following:

console.log(2==2);
console.log(2===2);

console.log(2=='2');
console.log(2==='2');

console.log(undefined == null);
console.log(undefined === null);

console.log(NaN == NaN);
console.log(NaN === NaN);

console.log([] == []);
console.log([] === []);

console.log([1, 2, 3] == [1, 2, 3]);
console.log([1, 2, 3] === [1, 2, 3]);

console.log({} == {});
console.log({} === {});

Arrays

What are the different ways to define array in Javascript?

What are the different ways to iterate through array in Javascript?

What's the difference between slice and splice methods?

Define a repeat function on array which which can be invoked like below:

var array = [1, 2, 3];
array.repeat(2); // return [1, 1, 2, 2, 3, 3];
array.repeat(3); // return [1, 1, 1, 2, 2, 2, 3, 3, 3];
[2, 3].repeat(1) // return [2, 3]

What is the use of map, filter, every, some and reduce method in Javascript?

Objects

What are the different ways to define objects in Javascript?

What is the difference between classical inheritance and prototypal inheritance?

Promise, Async/Await?

What is promise and how its different from callback?

Implement polyfill for promise

What is async/await?

Others

Is javascript a single threaded or multi-threaded language?

What is event loop and how it works?

What is the output of the following:

console.log(1);

console.log(2);

setTimeout(function() {
  console.log(3);
}, 0);

console.log(4);

What is the difference between async and defer attribute in script tag?