Data Structures and Algorithms

DSA Interview Questions

Top 30 JavaScript Interview Questions for Frontend Developers in 2025

Whether you’re gearing up for your first frontend role or aiming to level up in a competitive job market, nailing JavaScript interview questions is crucial. These questions often test your foundational knowledge, problem-solving skills, and ability to handle real-world scenarios. To get ahead, sign up for free resources and course updates that can supercharge your preparation.

JavaScript remains the backbone of frontend development, powering interactive web experiences across browsers. In 2025, with the rise of modern frameworks and performance demands, interviewers are focusing on both core concepts and advanced applications. This post draws from real questions asked in interviews at top companies, including FAANG, as reported in sources like InterviewBit and GeeksforGeeks. We’ll break down 30 high-quality questions—more than the typical top 20—to provide in-depth insights, code examples, and explanations. These are curated from actual interview experiences shared on platforms like Dev.to and GreatFrontEnd, ensuring they’re relevant and up-to-date.

If you’re looking to strengthen your overall skills, consider exploring structured learning paths. For instance, if data structures come up in coding rounds, our DSA course offers comprehensive training tailored for developers.

Why JavaScript Matters in Frontend Interviews

JavaScript isn’t just about syntax—it’s about understanding how it interacts with the DOM, handles asynchronous operations, and optimizes performance. According to a 2024 Stack Overflow survey, over 60% of developers use JavaScript daily, making it the most in-demand language for frontend roles. Interviewers at companies like Google and Amazon often probe these areas to assess your ability to build scalable, efficient applications.

In this guide, we’ll categorize questions into basic, intermediate, and advanced levels for easy navigation. Each includes a detailed explanation, code snippets, and tips to articulate your answer confidently. Remember, practice is key—try coding these out yourself.

Basic JavaScript Questions

These foundational questions test your core understanding and are commonly asked to freshers or in initial screening rounds.

1. What are the different data types in JavaScript?

JavaScript features primitive and non-primitive data types. Primitives include string, number, boolean, undefined, null, symbol, and bigint—immutable values passed by value. Non-primitives, like objects and arrays, are mutable and passed by reference.

For example:

				
					let num = 42; // Number (primitive)

let obj = { key: 'value' }; // Object (non-primitive)

console.log(typeof num); // "number"

Understanding this helps in avoiding bugs like unintended mutations in arrays. As per GeeksforGeeks, this is a staple in beginner interviews.

2. Explain hoisting in JavaScript.

Hoisting moves variable and function declarations to the top of their scope. var is initialized to undefined, but let and const enter a temporal dead zone (TDZ), causing errors if accessed early.

Example:

				
					console.log(x); // undefined

var x = 5;

console.log(y); // ReferenceError

let y = 10;

This behavior, highlighted in InterviewBit, can lead to subtle bugs if not managed with strict mode.

3. What’s the difference between == and ===?

== performs loose equality with type coercion, while === checks strict equality without coercion.

Example:

				
					console.log(5 == '5'); // true

console.log(5 === '5'); // false

Dev.to emphasizes using === to prevent unexpected results in frontend logic.

4. Difference between var, let, and const.

var is function-scoped and redeclarable; let and const are block-scoped. const can’t be reassigned, but its contents can mutate if an object. Table for clarity:

Keyword	Scope	Reassignable	Redeclarable
var	Function	Yes	Yes
let	Block	Yes	No
const	Block	No	No

From freeCodeCamp, this tests ES6 knowledge.

5. What is the DOM?

The Document Object Model (DOM) is a tree representation of HTML, allowing JavaScript to manipulate page structure dynamically. Example: document.getElementById(‘elem’).innerHTML = ‘New Text’;. GeeksforGeeks notes it’s essential for interactive UIs.

6. Difference between null and undefined.

undefined means a variable is declared but uninitialized; null is an intentional absence of value. Example: let a; console.log(a); // undefined let b = null;. Both equal with == but not ===.

7. What is implicit type coercion?

JavaScript automatically converts types during operations, like 5 + ‘5’ = ’55’. Avoid pitfalls by using strict checks, as advised in InterviewBit.

8. Explain the ‘this’ keyword.

this refers to the context of execution. In methods, it points to the object; globally, to window. Arrow functions lexically bind this. Example from freeCodeCamp.

9. What is event bubbling?

Events propagate from child to parent elements. Use event.stopPropagation() to halt. Crucial for event delegation in large apps.

10. What is NaN?

Not-a-Number, a special number value from invalid operations like 0/0. Check with isNaN().

Intermediate JavaScript Questions

These delve into practical applications, often seen in mid-level interviews.

11. What are closures?

A closure accesses outer scope variables post-execution.

Example:

				
					function outer() {

 let count = 0;

 return () => count++;

}

let increment = outer();

console.log(increment()); // 1

freeCodeCamp highlights their use in encapsulation.

12. Explain the event loop.

Manages async code by queuing tasks when the call stack is empty. Includes microtasks (Promises) and macrotasks (setTimeout). Dev.to explains its role in non-blocking I/O.

13. What are arrow functions?

Concise syntax without own this. Example: const add = (a, b) => a + b;. Not for methods.

14. What is lexical scope?

Scope determined by code position. Inner functions access outer variables.

15. How to clone an object?

Shallow: {…obj}; Deep: JSON.parse(JSON.stringify(obj)). Limits with functions/dates.

16. What is currying?

Transforming multi-arg functions into unary ones. Example:

const curryAdd = x => y => x + y;

console.log(curryAdd(2)(3)); // 5

17. Difference between apply, call, bind.

call invokes with args list; apply with array; bind returns new function.

18. What is IIFE?

Immediately Invoked Function Expression: (function() { … })();. For private scopes.

19. Explain async and defer in script tags.

async loads non-blocking; defer executes post-DOM parse. For optimization.

20. What is event delegation?

Attaching listeners to parents for efficiency. Uses bubbling.

Advanced JavaScript Questions

These test deep knowledge, common in senior or FAANG interviews.

21. What are generators?

Functions yielding multiple values: function* gen() { yield 1; yield 2; }. For iterators.

22. Difference between WeakSet/Set and WeakMap/Map.

Weak versions allow GC if keys are unreferenced; no size/iteration.

23. What are symbols?

Unique primitives for object keys: const sym = Symbol(‘key’);.

24. Explain Promise.all.

Settles array of promises; rejects on first error.

25. What is debouncing?

Delays function execution post-event. Example for search inputs.

26. What is throttling?

Limits execution rate, e.g., every 100ms.

27. How to prevent memory leaks?

Avoid unnecessary closures; nullify references.

28. What are proxies?

Customize object operations: new Proxy(target, handler).

29. Explain microtask queue.

Prioritizes Promises over macrotasks like setTimeout.

30. What is Shadow DOM?

Encapsulated DOM for components, hiding internals.

For more on web development fundamentals, check our web development course.

Preparation Tips for Frontend Interviews

Practice on platforms like LeetCode or GreatFrontEnd.
Build projects to apply concepts.
Review X discussions for recent trends.
For crash prep, try our crash course.
Master DSA and system design with master course.
Explore data science intersections via data science course.

Conclusion

Mastering these questions will boost your confidence. Practice regularly and stay curious. Ready to dive deeper? Sign up for updates and tackle your next interview head-on.

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Top 30 JavaScript Interview Questions for Frontend Developers in 2025

Why JavaScript Matters in Frontend Interviews

Basic JavaScript Questions

1. What are the different data types in JavaScript?

2. Explain hoisting in JavaScript.

3. What’s the difference between == and ===?

4. Difference between var, let, and const.

5. What is the DOM?

6. Difference between null and undefined.

7. What is implicit type coercion?

8. Explain the ‘this’ keyword.

9. What is event bubbling?

10. What is NaN?

Intermediate JavaScript Questions

11. What are closures?

12. Explain the event loop.

13. What are arrow functions?

14. What is lexical scope?

15. How to clone an object?

16. What is currying?

17. Difference between apply, call, bind.

18. What is IIFE?

19. Explain async and defer in script tags.

20. What is event delegation?

Advanced JavaScript Questions

21. What are generators?

22. Difference between WeakSet/Set and WeakMap/Map.

23. What are symbols?

24. Explain Promise.all.

25. What is debouncing?

26. What is throttling?

27. How to prevent memory leaks?

28. What are proxies?

29. Explain microtask queue.

30. What is Shadow DOM?

Preparation Tips for Frontend Interviews

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