Top 50 JavaScript Interview Questions
JavaScript Interview Questions and Answers
1. What are the different data types in JavaScript?
Primitive types: String, Number, BigInt, Boolean, undefined, Symbol, null. Non-primitive: Object, Array, Function.
2. What is the difference between var, let, and const?
var: function-scoped, hoistedlet: block-scoped, not hoistedconst: block-scoped, cannot be reassigned
3. What is hoisting?
Hoisting is JavaScript's default behavior of moving declarations to the top. var is hoisted with undefined; let and const are hoisted but not initialized.
4. What is the difference between == and ===?
==compares values after type coercion.===compares values and types strictly.
5. What is a closure?
A closure is a function that remembers variables from its outer scope even after the outer function has returned.
6. What is the difference between null and undefined?
undefined: variable declared but not assigned.null: assigned by the programmer to represent "no value".
7. What is the event loop in JavaScript?
It handles asynchronous callbacks. The call stack runs tasks, and the event loop pushes async tasks from the queue when the stack is clear.
8. What is a Promise?
A Promise is an object representing the eventual completion or failure of an async operation.
9. How does async/await work?
It simplifies working with Promises. await pauses execution until the Promise resolves or rejects.
10. What are arrow functions?
Concise function syntax: const fn = () => {}. They don't have their own this or arguments.
11. What is this in JavaScript?
this refers to the object that called the function. Its value depends on how the function is invoked.
12. What is a higher-order function?
A function that takes another function as an argument or returns a function.
13. What is currying?
Breaking down a function that takes multiple arguments into a series of functions that each take one argument.
14. What are the different ways to create an object?
Object literals
Constructor functions
Object.create()ES6 classes
15. What is the difference between slice() and splice()?
slice(start, end): returns a shallow copysplice(start, deleteCount, ...items): modifies original array
16. What is the difference between call, apply, and bind?
call: calls function with arguments individuallyapply: calls function with arguments as an arraybind: returns a new function with boundthis
17. What is the prototype chain?
Objects in JavaScript have a hidden [[Prototype]] property pointing to another object. This forms a chain for inheritance.
18. What is the difference between shallow copy and deep copy?
Shallow copy copies references to nested objects.
Deep copy clones all levels.
19. What is the difference between map, filter, and reduce?
map(): transforms array elementsfilter(): filters based on conditionreduce(): reduces to a single value
20. What are template literals?
String literals allowing embedded expressions: Hello, ${name}
21. What is destructuring?
Extracting values from arrays or objects into variables.
22. What is the spread operator (...)?
Expands iterable values into individual elements.
23. What is the rest operator?
Collects remaining elements into an array.
24. What is the difference between synchronous and asynchronous code?
Synchronous: executes line by line.
Asynchronous: continues without waiting for completion.
25. What are callbacks?
Functions passed as arguments to other functions to run later.
26. What is typeof?
Operator used to check the data type of a variable.
27. What is instanceof?
Checks if an object is an instance of a specific constructor.
28. What is NaN?
NaN stands for Not-a-Number. It results from invalid math operations.
29. How do you check if a value is an array?
Use Array.isArray(value).
30. What is event bubbling?
When an event starts from the deepest target element and bubbles up to ancestors.
31. How do you stop event propagation?
Use event.stopPropagation().
32. What is debouncing?
Delays execution of a function until after a wait time since the last call.
33. What is throttling?
Limits function execution to once in a specified interval.
34. What are modules in JavaScript?
Code separated into reusable files using import/export (ES Modules) or require/module.exports (CommonJS).
35. What is the difference between localStorage, sessionStorage, and cookies?
localStorage: no expirysessionStorage: cleared on tab closecookies: sent with every request
36. What are default parameters?
You can set default values for function parameters.
37. What is optional chaining (?.)?
Safely access nested properties without throwing errors if a property is null or undefined.
38. What is nullish coalescing (??)?
Returns the right-hand operand only if the left is null or undefined.
39. What is a symbol?
A unique and immutable primitive value often used as object keys.
40. What are WeakMap and WeakSet?
Similar to Map/Set but keys are weakly referenced and not enumerable.
41. What is a generator function?
A function that can be paused and resumed using function* and yield.
42. How do you handle exceptions in JavaScript?
Use try...catch blocks to handle runtime errors.
43. What is strict mode?
Adds stricter parsing and error handling to JavaScript.
44. What is tail call optimization?
An optimization technique where the last function call doesn’t add a new stack frame.
45. What is memoization?
Caching the results of expensive function calls to avoid re-computation.
46. What is the difference between eval() and Function()?
Both execute strings as code. eval() has access to scope; Function() creates new scope.
47. What are IIFEs?
Immediately Invoked Function Expressions: (function(){})()
48. What is object destructuring with default values?
const { a = 10, b = 20 } = obj;
49. What is the difference between for...in and for...of?
for...in: iterates over keysfor...of: iterates over values (iterables)
50. What is tail recursion?
A recursive function where the recursive call is the last operation. Optimized to avoid stack overflows.
