Top 20 JavaScript Coding Interview Questions for Programmers

Introduction

JavaScript is one of the most in-demand programming languages today. As a result, JavaScript coding interviews are an integral part of the hiring process for front-end, back-end, and full-stack developer roles.

Mastering common Top 20 JavaScript coding interview questions will help you stand out from other candidates and land the job. In this comprehensive guide, we share 20 of the best JavaScript interview questions and example solutions to boost your preparation.

JavaScript popularity has exploded in recent years, beyond just front-end development. Node.js brought JavaScript to the server. Frameworks like React and Angular transformed complex UIs. JavaScript now powers full-stack applications.

This growth means skilled JavaScript developers are in high demand. Acing the coding interview is key to unlocking new career opportunities.

JavaScript interview questions test your conceptual understanding of the language and its workings. Expect coding exercises and practical questions focused on:

• Language fundamentals – variables, functions, OOP prototypes etc.
• DOM manipulation
• Events and async handling
• Common algorithms
• Debugging and error handling
• Performance optimization
• Testing

Preparing responses beforehand helps you structure answers under pressure. Let’s explore examples of 20 top JavaScript interview coding questions.

20 JavaScript Coding Interview Questions

Here are 20 of the most common JavaScript interview questions for you to practice:

Q1. Difference between null and undefined

Example:

let num = null
let age 

console.log(num, age)

Answer:

• null is an assignment value that represents no object. It is assigned explicitly.
• undefined means a variable has been declared but not defined. It is implicitly assigned.

Q2. Difference between == and === operators

Example:

const a = 1
const b = '1'

console.log(a == b)
console.log(a === b)

Answer:

• == is abstract equality that coerces types to check equality.
• === is strict equality that checks type and value equality.

Q3. What is closure in JavaScript?

Example:

function greet(msg) {
  return function() {
    console.log(msg)
  }
}

const g1 = greet('Hello')
g1()

Answer: A closure is a function that remembers its outer variables and can access them. The inner function g1 can access the msg variable of the outer function greet.

Q4. What are implicit and explicit type conversions in JavaScript?

Example:

let a = '42'
let b = Number(a) // explicit
let c = a * 1 // implicit

Answer:

• Explicit conversion is manually converting using functions like Number() and String().
• Implicit conversion happens automatically like multiplying strings with numbers.

Q5. Compare var, let and const keywords for variable declaration

Example:

var a = 1
let b = 2
const c = 3

Answer:

• var has function scope, gets hoisted, and can be reassigned.
• let has block scope, no hoist, and can be reassigned.
• const has block scope, no hoist, and cannot be reassigned.

Q6. Explain how this keyword works in JavaScript

Example:

const user = {
  name: 'John',
  print: function() {
    console.log(this.name) 
  }
}

user.print()

Answer:

• this refers to the object calling the function. Here user.print() calls the function, so this refers to user.
• Arrow functions inherit this from the parent scope.

Q7. Create a function that generates a random number

Example:

function randomNum(min, max) {
  return Math.random() * (max - min) + min
}

Answer: We can use Math.random() and scale the number between min and max.

Q8. How do you clone an object in JavaScript?

Example:

const obj = { a: 1, b: 2 }

const shallowClone = {...obj} 

const deepClone = JSON.parse(JSON.stringify(obj))

Answer: Spread operator copies shallowly. JSON.parse(JSON.stringify(obj)) does a deep clone.

Q9. What is the event loop in JavaScript?

Answer: The event loop handles executing all asynchronous callbacks and tasks. It has a call stack, callback queue, and other APIs.

Q10. How do you compare two objects in JavaScript?

Example:

const obj1 = { foo: 'bar' }
const obj2 = { foo: 'bar' }

console.log(obj1 === obj2) // false

Answer: Object comparisons check the reference instead of value. Options are:

• Compare each key manually
• Use JSON.stringify()
• Iterate through keys to detect changes
• Use a library like lodash isEqual

Q11. What are the advantages of using Promises over callbacks?

Answer: Promises have several advantages over callbacks:

• Avoid callback hell with better code structure
• Handle asynchronous errors with .catch()
• Chain promises for sequence of operations
• Easier control flow for async logic
• Better handling of asynchronous dependencies

Q12. Explain prototypal inheritance

Example:

function Animal(name) {
  this.name = name
} 

Animal.prototype.walk = function() {
  console.log(`${this.name} is walking!`)
}

const dog = new Animal('Dog')
dog.walk()

Answer: Prototypal inheritance allows objects to inherit features from other objects via their prototype property. We can add shared methods to the prototype vs on each object instance.

Q13. What is the difference between synchronous and asynchronous code in JavaScript?

Answer:

• Synchronous code blocks further execution until it completes.
• Asynchronous code doesn’t block execution and callbacks handle any returned data.

Examples:

• alert() is synchronous
• setTimeout() is asynchronous

Q14. What is function hoisting in JavaScript?

Example:

catName('Tiger')

function catName(name) {
  console.log(name)
}

Answer:

• Function declarations are hoisted to the top before code execution.
• So function can be called before declaration.

Q15. How do you check if a key exists in an object?

Example:

const person = {name: 'John'}

'name' in person // true
person.hasOwnProperty('age') // false

Answer:

• Use in operator to check if key exists in object
• hasOwnProperty() checks if object has its own property vs inherited

Q16. What are JavaScript data types?

Answer: The built-in data types in JavaScript are:

• Primitive – Number, String, Boolean, Null, Undefined, Symbol
• Non-primitive – Objects, Arrays, Functions

Q17. How to iterate over objects in JavaScript?

Example:

const person = {
  name: 'John',
  age: 20 
}

// for..in loop
for (let key in person) {
  console.log(key, person[key])
}

// Object.keys
Object.keys(person).forEach(key => {
  console.log(key, person[key])  
})

Answer: Use for..in loop or Object.keys() to get keys of an object and iterate over them.

Q18. Explain variable scoping rules in JavaScript

Answer:

• Var – Functional scope, accessible within function
• Let – Block scope, accessible within {} block
• Const – Block scope

Q19. How do you handle errors in JavaScript?

Example:

try {
  // code that may throw error
} catch (err) {
  // handle error
}

Answer:

• Use try…catch blocks to handle errors gracefully
• throw manually throw errors
• Custom error classes can also be created

Q20. What are some methods used for looping arrays in JavaScript? (continued)

Example:

const colors = ['red', 'green', 'blue'] 

// forEach
colors.forEach(color => {
  console.log(color) 
})

// map 
const capitalColors = colors.map(color => {
  return color.toUpperCase()
})

// filter
const longColors = colors.filter(color => {
  return color.length > 3
})

Answer:

• for loop – iterate index
• forEach – iterate values
• map – transform array
• filter – filter array

And many more methods like reduce, find, every etc.

Conclusion

These 20 coding interview questions and example solutions provide a solid foundation to prepare for the programming portion of your JavaScript job interview.

Practice these questions until you can easily explain the concepts and code good solutions quickly. Pay attention to code quality, edge cases and performance.

A strong grasp of JavaScript fundamentals coupled with problem-solving skills will help you succeed at technical interviews and land your dream role!

Frequently Asked Questions

Here are some common questions about JavaScript coding interviews:

What other topics should I review besides coding questions?

Also ensure you brush up on JavaScript concepts like scopes, context, prototypes, async/await, strict mode, callbacks etc. Know basics of tools like Node, npm, webpack, and ES6 modules.

What types of projects demonstrate JavaScript skills to interviewers?

Build sample full-stack apps with Node + frontend framework like React or Vue. Contribute to open source JavaScript projects on GitHub. These showcase both coding skills and initiative.

How important is performance optimization during interviews?

While brute force solutions are fine initially, you will gain bonus points by optimizing memory usage, removing duplicates, caching etc. Explain your thought process.

What if I’m stuck on a question during the interview?

Don’t panic. Explain your initial approach and thought process. Interviewers want to see how you think through problems. Ask relevant clarifying questions and think aloud.

How much time should I allocate for studying JavaScript before interviewing?

Aim for 2-3 weeks of dedicated practice. Solve coding challenges, review projects, research companies and roles. Refine your personal portfolio. Internalize 20-30 top interview questions.