Ultimate List of JavaScript Interview Questions. 90 questions with answers

JavaScript Question № 1 Practical for Senior

How to compare two objects with nested properties in JavaScript?

JavaScript Question № 2 Theoretical for Middle

List special numeric values.

JavaScript Question № 3 Theoretical for Senior

Explain the Map , Set , WeakMap and WeakSet .

JavaScript Question № 4 Theoretical for Middle

List the methods to search in an array.

JavaScript Question № 5 Theoretical for Middle

What is the purpose of the file package - lock . json ?

JavaScript Question № 6 Theoretical for Junior

What’s the difference between using “ var ” , “ let ” , and “ const ” ?

JavaScript Question № 7 Theoretical for Middle

What is Cross-Origin Requests and CDN?

JavaScript Question № 8 Practical for Middle

Does it make sense to add directive ‘ use strict ’ in modern scripts?

JavaScript Question № 9 Theoretical for Middle

What are web workers?

//sample
let person = {
  name: 'Anna',
  age: 56,
  job: {
    company: 'Tesco',
    title: 'Manager'
  }
};
function getInfo(person) {
  var age = person.age,
    yearOfBirth = 2018 - age,
    name = person.name,
    company = person.job.company;
  console.log(`${ name } works at ${ company } and was born in ${ yearOfBirth }.`);
}

JavaScript Question № 11 Practical for Middle

Write an example of testing the sum function.

JavaScript Question № 12 Theoretical for Junior

How to show the user a notification that javascript is not enabled by his browser?

JavaScript Question № 13 Practical for Senior

Give webpack config sample for scss using.

JavaScript Question № 14 Theoretical for Junior

Why is the order of adding scripts to the page important?

JavaScript Question № 15 Practical for Senior

Create a script to lazy load the images shown after scrolling.

JavaScript Question № 16 Theoretical for Junior

How to get the current URL?

//Sample
const developer = new DeveloperBuilder('John')
  .addSkill('ES6')
  .addSkill('TypeScript')
  .setFramework('React');

JavaScript Question № 18 Practical for Senior

Create a rectangle object with a private width and public height using the Revealing Module Pattern.

// add the code here
setRate(5); // 5
setRate(3); // 4
setRate(4); // 4
setRate(0); // 3

JavaScript Question № 20 Practical for Senior

Write Observer pattern sample.

//Sample
 
function Person(name, lastName) {
  this.name = name;
  this.lastName = lastName;
}
 
Person.prototype.getFullName = function() {
  return this.name + " " + this.lastName;
};
 
function Developer(skills, name, lastName) {
  this._super.call(this, name, lastName);
  this.skills = skills;
}
 
Developer.prototype = Object.create(Person.prototype);
Developer.prototype.constructor = Developer;
Developer.prototype._super = Person;
 
Developer.prototype.getDeveloperInfo = function() {
  return this.name + " " + this.lastName + ", skills: " + this.skills;
};
 
var developer = new Developer("JavaScript, React, Redux", "John", "Doe");
console.log(developer.getFullName());
console.log(developer.getDeveloperInfo());

i = 10;
var j = 1;
for(var i=1; i < 5; i++) {
  j++;
}
console.log(i);
console.log(j++);

// store sum, evenSum and evenFlag
// create an example
 
function getOddOrEvenSum(arr) {
  var a = 0;
  var b = 0;
 
  arr.forEach(function(number) {
    // place for code
  });
  // place for code
}
 
var arr = [1, 2, 3, 4, 5, 10, 11];
console.log(getOddOrEvenSum(arr))

const arr = [11, 29, 5]

JavaScript Question № 25 Practical for Middle

Write a JavaScript function to check that two words are anagrams (formed by rearranging the letters of another)

var arr = [1, 2, 3, 4, 5];
var sum = arr.sum();
console.log(sum)

JavaScript Question № 27 Practical for Junior

How to check that variable is not undefined? How to check that a property exists in an object?

// example with parameters in which strict comparison is important
 
function checkTheYear(initYear, targetYear) {
 switch(targetYear) {
   case initYear - 30:
     console.log('I\'m sure in 1985, plutonium is available at every corner drugstore, but in 1955 it\'s a little hard to come by.');
     return true;
    case initYear - 100:
     console.log('A hundred years ago? But that\'s now!');
     return true;
    case initYear + 30:
     console.log('Nobody calls me chicken!')
            return true;
    default:
     console.log('Time traveling is just too dangerous. Better that I devote myself to study the other great mystery of the universe: women!');
     return false;
  }
}
 
function backToTheFuture(targetYear) {
  var timeMachine = {
    model: 'DeLorean DMC-12',
    initYear: '1985'
  }
  if(checkTheYear(timeMachine.initYear, targetYear)) {
   // execute the secret code for time travel to the targetYear
  }
}

let string = "Soshace", substring = "sh"

JavaScript Question № 30 Theoretical for Senior

How to forbid adding, removing and changing of object properties?

function Developer(experience) {
  this.experience = experience;
}
Developer.prototype = {
  skills: ['JavaScript']
};
 
let senior = new Developer(7);
let junior = new senior.constructor(1);
 
console.log( junior.experience ); // undefined

JavaScript Question № 32 Practical for Senior

Write a recursive function to create a deep copy of the object

JavaScript Question № 33 Practical for Middle

Give code samples for immutable operations using Object . assign and Spread operator

<button id="click-me" class="btn">Click Me</button>
<script type="text/javascript">
var elements = {
    button: document.getElementById('click-me'),
};
elements.button.onclick = function() {
    alert('clicked!');
    document.body.removeChild(elements.button);
}
</script>

JavaScript Question № 35 Practical for Junior

Write a simple function to check that a number is an integer?

function getFullPrice(type) {
    var ticket = 10,
    luggage = 7,
    service = 5,
    taxes = 3,
    price = 0;
 
    switch(type) {
        case 'ticket':
     price = ticket;
     break;
        case 'luggage':
     price = ticket + luggage;
     break;
        case 'service':
     price = ticket + luggage + service;
     break;
  }
  
  price += taxes;
  
  return price;
}
console.log(getFullPrice('ticket'));
console.log(getFullPrice('luggage'));
console.log(getFullPrice('service'));

JavaScript Question № 37 Practical for Senior

Provide some shorthand coding techniques using operators “&&” , “ | | ” and “ ? ”

JavaScript Question № 38 Theoretical for Junior

How to stop the setInterval function?

console.log(1.1 - 0.2)
// console output
0.9000000000000001

function myFunc() {
  // some logic
}
and
var myFunc = function() {
 // some logic
}

JavaScript Question № 43 Theoretical for Middle

What is a hasOwnProperty ?

JavaScript Question № 44 Theoretical for Senior

Explain the Symbol type.

JavaScript Question № 45 Theoretical for Senior

What are accessor descriptors?

let user = {
  name: "John",
  logName() {
    console.log(`My name is ${this.name}!`);
  }
};
 
setTimeout(user.logName, 1000);

JavaScript Question № 47 Theoretical for Senior

What is the difference between Boolean ( “ true ” ) and new Boolean ( “ true ” ) ?

JavaScript Question № 49 Theoretical for Senior

Categorize JavaScript test tools by type.

function removeElements(arr, criteriaFunc) {
  var i;
  for (i = 0; i < arr.length; i++) {
    if (criteriaFunc(arr[i])) {
      // place for code
    }
  }
  return arr;
}
 
console.log(removeElements([1, 2, 5, 4, 6, 7, 3, 1, 5], function(number) {
  return number < 5;
}));

JavaScript Question № 54 Theoretical for Middle

How does prototype work?

JavaScript Question № 55 Theoretical for Junior

What is the scope?

JavaScript Question № 56 Theoretical for Middle

Explain static properties and methods.

arraySum([[1, 2, [3, 4]], [9], [10, 12]])

JavaScript Question № 58 Theoretical for Middle

How to redirect the browser to another page?

JavaScript Question № 59 Theoretical for Senior

What are bubbling, capturing, and event delegation in the browser?

JavaScript Question № 61 Theoretical for Middle

What is the difference between import and require?

JavaScript Question № 62 Practical for Senior

Write a code to drag and drop an absolute positioned element.

JavaScript Question № 63 Theoretical for Middle

What are the benefits of debugging the script in the browser?

JavaScript Question № 64 Practical for Senior

How to set an environment variable in the npm script?

{
    "parserOptions": {
        "ecmaVersion": 6,
        "sourceType": "module",
        "ecmaFeatures": {
            "jsx": true
        }
    },
    "rules": {
        "semi": "error"
    }
}

JavaScript Question № 66 Theoretical for Senior

For what purposes can the global object console be used, besides logging?

JavaScript Question № 67 Theoretical for Middle

What useful plugins / extensions / features can be added in IDE?

JavaScript Question № 68 Theoretical for Middle

What is a destructuring assignment? Give an example.

JavaScript Question № 69 Theoretical for Senior

What is yield, give an example?

const printSec = (number, callback) => {
  setTimeout(() => {
      console.log(`${number} sec`)
      callback()
    },
    1000)
}
printSec(1, () => {
  printSec(2, () => {
    printSec(3, () => {})
  })
})

JavaScript Question № 71 Theoretical for Senior

List examples in which ECMAScript languages ​​are used other than the browser and the web backend.

JavaScript Question № 72 Theoretical for Middle

What is the difference between REST API and GraphQL?

JavaScript Question № 73 Theoretical for Middle

What is PWA?

JavaScript Question № 74 Theoretical for Middle

What is SSR?

JavaScript Question № 75 Theoretical for Middle

What is Static Site Generators?

JavaScript Question № 76 Theoretical for Senior

List types of storage in the browser.

JavaScript Question № 77 Theoretical for Middle

What is a common practice for type checking in JavaScript?

JavaScript Question № 78 Theoretical for Middle

What is Lodash?

JavaScript Question № 79 Theoretical for Middle

What is a Hybrid Mobile App?

JavaScript Question № 80 Theoretical for Middle

What are the disadvantages of using jQuery with SPA?

JavaScript Question № 81 Theoretical for Middle

List some popular libraries for data visualization.

JavaScript Question № 82 Theoretical for Senior

What are the libraries for state management?

JavaScript Question № 83 Theoretical for Junior

List SPA frameworks and libraries.

JavaScript Question № 84 Theoretical for Middle

List the libraries to making requests from the browser and Node.js?