Top JavaScript Interview Questions and Answers You Must Know

JavaScript is a popular programming language, used everywhere from simple HTML scripts to complex browser apps, making websites feel dynamic and interactive. It’s also the foundation for popular frameworks like React, Angular, and Vue.

Getting ready for a JavaScript developer interview means understanding the basics, practicing with real problems, and proving you can tackle coding challenges with confidence.

Employers want developers who can handle advanced coding challenges. Good preparation helps you explain concepts clearly and tackle problems confidently during a function call or coding task.

Essential JavaScript Interview Questions for Beginners

What are primitive data types in JavaScript?

A primitive data type is a type that isn’t an object and doesn’t have any properties or methods.

String: A sequence of characters, like “Hello”.
Number: Whole numbers or decimals, like 42 or 3.14.
Boolean: True or false values.
Undefined: A variable that’s declared but not given a value.
Null: Intentionally empty or “no value.”
Symbol: unique and immutable identifiers introduced in the ES6 version of JavaScript.
bigint: for arbitrarily large integers beyond the safe integer limit

Unlike reference types like objects, arrays, and functions, primitive values are stored directly in the variable itself, not as references.

let name = “Alex”; // string (primitive)

let person = { name: “Alex” }; // object (reference)

What is the difference between var, let, and const?

In JavaScript programming, variable declaration affects the scope of a variable and how it behaves during execution:

var: Function-scoped, can be redeclared, and gets hoisted—but it can sometimes cause bugs in bigger codebases.
let: Block-scoped, can’t be redeclared in the same scope, and was introduced in ES6.
const: Block-scoped, cannot be reassigned after initialization.

Temporal Dead Zone (TDZ)

Variables declared with let and const exist in a TDZ from the start of the block until their declaration is processed, preventing access before initialization.

What is hoisting in JavaScript?

Hoisting is a behaviour in which the JavaScript engine moves variable and function declarations to the top of their scope before code execution. This means you can technically call a JavaScript function before it appears in the code, although the way it works depends on how the variable or function is declared.

Function declarations are fully hoisted, meaning both the function name and its implementation are available before the code is executed.
Variables in JavaScript declared with var are hoisted but initialized with undefined.
Declarations using let and const are hoisted but remain in the temporal dead zone until the execution reaches their line of code.

console.log(myVar); // undefined (due to hoisting of var)
var myVar = 5;

myFunc(); // Works because function declaration is hoisted
function myFunc() {
console.log(“Hello from hoisted function”);
}

What is strict mode in JavaScript?

Strict mode is a feature used in JavaScript to enforce stricter parsing and error handling in your code. You can turn it on by adding the ‘use strict’ directive right at the top of your JavaScript file, inside a function, or even inside a script tag in your HTML.

Purpose of the 'use strict' directive

Disallows duplicate parameter names in a function.
Makes debugging easier by catching silent errors that would otherwise go unnoticed.
Eliminates some implicit type coercion in JavaScript cases that can cause unexpected results.
Prepares code for the next JavaScript standards and optimizations by encouraging better coding habits.

‘use strict’;
x = 10; // ReferenceError: x is not defined

Strict mode is especially important in large JavaScript applications or when integrating with JavaScript frameworks, as it ensures a more predictable and maintainable codebase.

What is a closure in JavaScript?

A closure is created when a JavaScript function remembers its lexical scope even after that function is executed outside its original scope. This happens because functions in JavaScript form a chain of scopes, and the inner function can “close over” variables from the outer function.

Lexical scoping means a function can access variables from the scope in which it was defined, not where it is executed.
In JavaScript in a browser or external JavaScript files, closures behave the same way because scope is determined at declaration time.
A keyword in JavaScript refers to specific reserved words like function or const that play a role in how scopes and closures are defined.

function makeCounter() {
let count = 0;
return function () {
count++;
return count;
};
}

const counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2

What are functions in JavaScript?

Functions are one of the most essential building blocks in JavaScript, enabling reusable, modular, and maintainable code. In interviews, common questions about functions test your understanding of declaration methods, scope, and execution.

Function declarations vs function expressions

Function declaration: Fully hoisted, meaning it can be called before its definition.
Function expression: Assigned to a variable; only available after the variable is initialized.
In both cases, you can place them in an external JavaScript file or directly in an HTML file to run in the browser.

Arrow functions and their lexical this binding

Arrow functions don’t have their own this value — they inherit this from the enclosing scope.
Ideal for callbacks where preserving context is important.

Immediately Invoked Function Expressions (IIFE)

A function that runs immediately after being defined.
It’s handy for keeping variables private and avoiding name clashes, especially when dealing with lots of JavaScript code in big projects.

Intermediate JavaScript Interview Questions

What is prototypal inheritance in JavaScript?

Prototypal inheritance is a core feature of JavaScript, where objects can inherit properties and methods from other objects through their prototype chain. This lets you reuse code without copying the same methods into every instance.

Every Object in JavaScript has an internal link to another Object called its Prototype.
If a property or method isn’t found on the object itself, JavaScript looks for it in the prototype chain.
This mechanism is more flexible than fixed inheritance structures, making it ideal for dynamic applications.

Difference between classical inheritance and prototypal inheritance

Classical inheritance (as in Java or C#) uses classes to define blueprints for objects.
Prototypal inheritance is instance-based — objects inherit directly from other objects without rigid class structures.
JavaScript now supports class syntax (introduced in ES6), but under the hood, it still uses prototypes.

const animal = { speak() { console.log(“Animal sound”); } };
const dog = Object.create(animal);
dog.speak(); // “Animal sound” (inherited from prototype)

What are Constructor Functions?

A constructor function is a special type of function used to create multiple objects with the same structure and behaviour.

By convention, constructor functions start with a capital letter.
Using the new keyword makes a new object, links its prototype to the constructor’s prototype property, and runs the constructor’s code.
Any methods or properties you add to the constructor’s prototype are then shared by all objects made from it.

How does the event loop work in JavaScript?

JavaScript is a single-threaded language, meaning it executes one task at a time. However, it can still handle asynchronous operations efficiently through the event loop mechanism.

While synchronous code runs line by line, asynchronous tasks (like setTimeout or fetching data) are handled outside the main thread and queued for later execution.

Call stack: Where JavaScript keeps track of the functions currently being executed.
Callback queue: Holds functions ready to be executed after the stack is clear.
Microtask queue: Holds promises and other microtasks, which are processed before the callback queue.

What are higher-order functions in JavaScript?

Higher-order functions are JavaScript functions that either take other functions as arguments, return functions as results, or do both. They’re a core idea in functional programming and are super popular in JavaScript for writing cleaner, reusable code.

Allows passing behaviour (functions) into other functions for flexible execution.
Useful for abstraction, reducing code duplication, and improving maintainability.

Examples such as map, filter, and reduce

map: Changes each element in an array using a callback function you provide.
filter: Creates a new array with only the elements that pass a certain condition.
reduce: Accumulates values in an array into a single result.

What is type coercion in JavaScript?

Type coercion in JavaScript is when a value gets converted from one data type to another, either automatically (implicitly) or on purpose (explicitly).

Implicit coercion happens automatically, like when you add a number to a string and JavaScript turns the number into text.
Explicit coercion is when you do it yourself using methods like Number(), String(), or Boolean().

How JavaScript converts types during operations

In + operations, if one operand is a string, JavaScript converts the other operand to a string.
In mathematical operations (other than +Operands are usually converted to numbers.

console.log(5 + “5”); // “55” (implicit, number → string)
console.log(Number(“5”) + 5); // 10 (explicit, string → number)

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

In JavaScript, == is the equality operator, while === is the strict equality operator.

Equality operator (==)

Compares values for equality after performing type coercion.
Can lead to unexpected results due to implicit type coercion.

Strict equality operator (===)

Compares both value and type without converting types.
More predictable and recommended in most cases.

Advanced JavaScript Interview Questions

Explain object-oriented programming in JavaScript

JavaScript is an object-oriented programming language, but unlike class-based languages such as Java or C#, it is prototype-based. Objects can directly inherit from other objects through the prototype chain, making the language flexible and dynamic.

Instead of strict class hierarchies, JavaScript uses prototypes to share properties and methods between objects.
ES6 introduced class syntax, but under the hood, it still uses prototype-based inheritance.

Key OOP concepts in the JavaScript context

Encapsulation: Grouping related data (properties) and behaviour (methods) into a single unit, often achieved with closures or private fields.
Inheritance: One object can grab properties and methods from another using its prototype.
Polymorphism: The same method name can behave differently depending on which object is using it, usually done by overriding the method.

class Animal {
speak() {
console.log(“Animal sound”);
}
}
class Dog extends Animal {
speak() {
console.log(“Woof!”);
}
}
new Dog().speak(); // Woof!

What are JavaScript design patterns?

JavaScript design patterns are tried-and-true ways to solve common programming problems. They help structure code in a way that is scalable, maintainable, and easier to understand.

Module Pattern: Encapsulates related code and exposes only necessary parts, often using closures.
Observer Pattern: Allows objects to subscribe to events and be notified when changes occur (common in UI frameworks).
Singleton Pattern: Makes sure there’s only one instance of a specific object throughout the whole application.

What are asynchronous operations?

Asynchronous operations allow JavaScript to perform tasks without blocking the main thread. This is essential for tasks like API requests, file reading, or timers.

Core asynchronous techniques

Callback functions: Functions passed as arguments to be executed later.
Promises: Objects that represent a value that might be available now, later, or maybe never.
Async/await: Syntax sugar over promises for writing asynchronous code in a synchronous style.

What is the difference between synchronous and asynchronous JavaScript code?

JavaScript can execute code synchronously or asynchronously, and understanding the difference is critical for writing efficient, responsive applications.

Synchronous (blocking): Code runs line by line, with each statement waiting for the previous one to finish before executing.
Asynchronous (non-blocking): Some operations can run in the background, letting the rest of the code keep going without having to wait.

console.log(“Start”);
setTimeout(() => console.log(“Async task complete”), 1000);
console.log(“End”);
// Output: Start → End → Async task complete

How does JavaScript handle function scope and closures?

JavaScript uses function scope and lexical scoping to determine variable accessibility, and closures extend the lifetime of those variables beyond their original scope.

A variable’s scope is defined by where it’s declared in the code.
The scope chain is the hierarchy of variable access from inner functions outward to the global scope.
Lexical scoping means that functions remember the scope in which they were created, not where they are called.

function secret() {
let hidden = “top-secret”;
return function() {
return hidden;
};
}
const getSecret = secret();
console.log(getSecret()); // “top-secret”

What is the prototype chain?

The prototype chain is JavaScript’s mechanism for property and method delegation between objects.

When you access a property on an object, JavaScript first checks if that property exists directly on the object itself.
If not found, it checks the object’s prototype, and then continues up the chain until it reaches Object.prototype or null.
All objects have an internal link to their prototype ([[Prototype]]).
Prototypes can themselves have prototypes, forming the chain.

const animal = { eats: true };
const dog = Object.create(animal);
console.log(dog.eats); // true (found in prototype)

How does ES6 improve JavaScript coding?

The ES6 (ECMAScript 2015) update introduced features that made JavaScript more powerful, maintainable, and expressive.

let and const for block-scoped variable declarations.
Arrow functions with lexical this binding.
Template literals for easy string interpolation.
Destructuring assignment for arrays and objects.
Class syntax for clearer OOP-style code.
Modules (import/export) for better code organization.

How can you manipulate the DOM?

JavaScript provides built-in methods for Document Object Model (DOM) manipulation, enabling developers to dynamically change webpage content and behaviour.

Selecting elements: document.getElementById(), document.querySelector(), and document.querySelectorAll() allows direct element access.
Adding/removing nodes: appendChild(), removeChild(), and createElement() let you modify the DOM structure.
Event handling: addEventListener() Attaches event handlers to elements.

document.querySelector(“#btn”).addEventListener(“click”, () => {
document.querySelector(“#message”).textContent = “Button clicked!”;
});

Final Thoughts

Employers value candidates who can do more than just write code—they want developers who can explain their thinking, back up their decisions, and make technical concepts easy for others to understand. Writing clean, efficient, and maintainable code shows professionalism and foresight, while being able to handle unexpected challenges on the spot shows adaptability and problem-solving skills.

To get there, keep practicing regularly, experiment with different approaches, and test yourself on both familiar and unfamiliar problems. Build a strong understanding of key concepts like scope, closures, and asynchronous code, while also exploring advanced patterns and ways to optimize performance. Looking for your next developer job? Nail your JavaScript interviews and get hired faster. Check out top openings today.

Frequently Asked Questions

How to Prepare for Your JavaScript Developer Interview

Getting ready for a JavaScript developer job is about knowing the concepts, putting them into practice, and showing you can solve problems effectively.

Understand the fundamentals: Review core concepts like variables, data types, functions, and scope to confidently tackle any JS interview.

Practice problem-solving: Use coding challenges to sharpen your ability to write clean, efficient solutions under time constraints.

Study common topics: Familiarize yourself with closures, prototypal inheritance, and event loops, as these interview questions will help you address real-world coding problems.

Build small projects: Apply concepts by building mini-apps to demonstrate both your technical knowledge and practical skills.

Review real interview scenarios: Simulate your next JavaScript interview with mock sessions to improve your communication and thinking speed.

How to Approach JavaScript Interview Questions and Answers Effectively?

Answering JavaScript interview questions effectively means understanding the “why” behind each answer, not just the “what.”

Analyze the question first: Break down the problem into smaller parts before jumping to code.

Relate to real-world examples: If asked about prototypes, illustrate with a practical object inheritance scenario.

Show your reasoning: Walk through your thought process so reviewers can see your critical thinking—JavaScript often has multiple valid solutions.

Balance theory with code: Pair short, working code snippets with your explanations to make concepts concrete and easy to follow.

Stay adaptable: Be ready to switch approaches if the interviewer suggests constraints or alternative methods.

What Tools and Resources Help You Practice Coding for Interviews?

Having the right tools and resources can make your preparation faster, more focused, and more effective.

Online coding platforms: Use sites like LeetCode, HackerRank, or Codewars to get varied challenges.

JavaScript documentation: MDN Web Docs is a reliable source for syntax, APIs, and examples of object-oriented programming language features.

Version control practice: Work with Git and GitHub to manage code, as collaboration skills are often tested indirectly.

Local coding environment: Set up VS Code with extensions for linting, debugging, and formatting to mirror professional workflows.

Video tutorials & courses: Platforms like Frontend Masters or freeCodeCamp help deepen understanding through guided lessons.

How to Demonstrate Your Understanding of JavaScript Best Practices?

Employers value developers who follow best practices that make code reliable, readable, and easy to maintain.

Write clean code: Use proper naming, keep your indentation neat, and don’t overcomplicate things.

Use modern features wisely: Use ES6+ features when they fit, but always double-check that they work smoothly in your project’s setup.

Optimize for performance: Reduce DOM manipulations, cache values, and minimize loops when possible.

Explain your design choices: Justify why you used a certain pattern or structure during a Java and JavaScript discussion.

Document and test: Maintain clear comments, write unit tests, and ensure code behaves as expected under different conditions.

What Are the Top JavaScript Interview Questions You Must Know?

Certain core topics appear frequently in interviews because they test both fundamental and applied knowledge.

Scope and closures: Understand how variable accessibility works and how closures preserve state.

Prototypal inheritance: Explain how objects inherit properties and methods through the prototype chain.

Asynchronous programming: Demonstrate knowledge of callbacks, promises, and async/await.

Type coercion and equality: Distinguish between == and ===, and explain implicit vs. explicit conversions.

Event loop mechanics: Show you understand how JavaScript handles synchronous and asynchronous tasks for smooth browser performance.