Basic JavaScript Interview Questions & Answers – Part 1

Explore 25 carefully selected JavaScript interview questions covering Core Concepts, Functions and Scope, Objects & Arrays, and Asynchronous JavaScript. This beginner-friendly guide helps you build a strong foundation and confidently tackle real-world coding interviews.

🧠 Core Concepts

1. What is JavaScript?
JavaScript is a lightweight, interpreted programming language used to create interactive effects within web browsers.
2. What are the data types in JavaScript?
StringNumberBooleanUndefinedNullSymbolBigIntObject
3. What is the difference between == and ===?
== checks value only (type coercion).
=== checks value and type.
4. What is undefined vs null?
undefined : variable declared but not assigned.
null : intentionally assigned empty value.
5. What are var, let and const
var : function-scoped.
let : block-scoped.
const : block-scoped and immutable reference.
6.What is hoisting?
JavaScript's default behavior of moving declarations to the top of the scope.
7.What is a closure?
A function that remembers variables from its lexical scope even when executed outside that scope.
8.What is a callback?
A function passed as an argument to another function to be executed later.
9.What is the event loop?
It handles async operations by using a queue to manage callbacks and ensures non-blocking behavior.
10.What is the difference between call, apply, and bind?
call : calls a function with a given this and arguments.
apply : same as call, but arguments are passed as an array.
bind : returns a new function with bound this.

🔁 Functions and Scope

11. What are arrow functions?
Shorter function syntax. Does not have its own this, arguments, or super.
12. What is lexical scope?
Scope defined by the location of variable declarations in the source code.
13. Difference between function declaration and expression?
Declarations are hoisted; expressions are not.
14. What is IIFE (Immediately Invoked Function Expression)?
A function executed immediately after its creation:
```
(function() { console.log('IIFE'); })();
```
15. What is the spread operator (...)?
Expands iterable elements:
```
let arr = [1, 2]; let newArr = [...arr, 3];
```

🧩 Objects & Arrays

16.How to clone an object?
```
let clone = {...original}; // shallow
```

17.What is destructuring?

let [a, b] = [1, 2]; let {name} = {name: "John"};

18.Difference between shallow and deep copy?
Shallow: references nested objects Deep: clones everything recursively
19.What is this, in JS?
Refers to the object that is executing the function.
20. What is prototype chain?
Objects inherit properties from another object via __proto__.

⚙️ Asynchronous JavaScript

21. What is a promise?
An object representing eventual completion or failure of an async operation.
22. What are async/await ?
Syntax sugar over promises for better readability.

23. How to handle errors in async code?

try { await something(); } catch (e) { console.error(e); }

24. What is the use of setTimeout and setInterval?
Delays or repeatedly executes a function after time delay.
25. Difference between synchronous and asynchronous?
Sync: executes line-by-line

Async: continues running without waiting

Continue with Javascript Questions Part - 2