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Halil Can Ozcelik
Halil Can Ozcelik

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JavaScript Interview Coding Questions — 1

Online coding is a crucial part of software developer job interviews. In this article, I will try to add several coding questions. These will be evaluation focused code snippets instead of task based coding questions. Additionally, I will share online code links at the end of every question.

Let’s start with one of the most well-known one:

  1. In this question, closure and asynchronous code handling knowledge is assessed.

    // What will be the console log of the code below?
    for (var i = 0; i < 4; i++) {
      setTimeout(() => console.log(i), 0);

    It will write 4, 4, 4, 4 in console because setTimeout() is a
    asynchronous function and it will be executed after for cycle is completed. i is defined outside scope of for loop and it will be equal to 4 when console.log() starts to write.
    How can you fix it to write 0, 1, 2, 3? Here is the possible solutions:

    // Solution 1:
    for (let i = 0; i < 4; i++) {
      setTimeout(() => console.log(i), 0);
    // Solution 2:
    for (var i = 0; i < 4; i++) {
      (function (i) {
        setTimeout(() => console.log(i), 0);

    Solution 1: You can change declaration of i from var to let
    because let is block scoped variable but var is function scoped variable.
    Solution 2: You can wrap setTimeout() with a function to limit
    the scope of i variable. But you should pass i as parameter to your IIFE (Immediately-invoked Function Expression).

    You can test it in below:

  2. This one is about scope of this.

    // What will be the logs in console?
    function nameLogger() { = 'halil';

    Console output will be following:

    second: undefined
    first: halil
    third: halil

    While “second” is executed is undefined but after nameLogger() function is executed it will be defined. Because, this refers to the global object in a function. So the others log "halil" as expected.
    !! It will not work with 'use strict' because in a function, in strict mode, this equals undefined instead of global object.

    You can test it in below:

  3. The last one is about this and bind

    // What will be written?
    const module = {
      x: 55,
      getX: function () {
        console.log('x:', this.x);
    const unboundX = module.getX;

    Console output will be x: undefined because this refers to current owner object. So when you assign only getX method to a new object unboundX, this will refers to it. And it has no x property, that’s why this.x equals to undefined.
    You can fix it by using bind() as below:

    const boundX = unboundX.bind(module);

    Now you bind module object to unboundX function. Then it can reach the x property of module.

    You can test it in below:

You can read some of my other articles from the links below:

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