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Important Javascript Questions And Answers For Beginners

by Digyfindy

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JavaScript Questions and Answers
JavaScript Interview Questions
JavaScript Basics
JavaScript for Beginners
Web Development Tips
Coding Interview Prep
Frontend Development
Learn JavaScript Easily
JavaScript Concepts Simplified

1. Purpose of JavaScript

JavaScript makes web pages come alive. HTML builds the structure, CSS makes it look nice, and JavaScript adds brainpower—like buttons that respond when you click them, dropdowns, or games running in the browser.

2. Key Features

  • Lightweight and easy to learn

  • Runs directly in the browser

  • Supports both object-oriented and functional programming

  • Asynchronous (doesn’t always wait in line, which is why you can fetch data without freezing the page)

3. Data Types

Two categories:

  • Primitive: string, number, boolean, null, undefined, symbol, bigint

  • Non-primitive: objects, arrays, functions

👉 Example:

 
let name = "Alex"; // string let age = 25; // number let isCool = true; // boolean

4. Closure

A closure happens when a function “remembers” the variables from its outer scope, even after that scope has gone away.

 
function outer() { let count = 0; return function inner() { count++; return count; }; } const counter = outer(); console.log(counter()); // 1 console.log(counter()); // 2

5. == vs === Operators

  • == checks only values (loose equality).

  • === checks both value and type (strict equality).

 
5 == "5"; // true 5 === "5"; // false

6. Difference Between Null and Undefined

  • undefined → a variable declared but not assigned.

  • null → an intentional empty value.

7. Event Delegation

Instead of attaching event listeners to every element, you attach one to the parent and handle children via event.target. Saves memory and keeps code clean.

8. Callback

A callback is a function passed into another function to run later.

 
function greet(name, callback) { console.log("Hello " + name); callback(); } greet("Sam", () => console.log("Welcome!"));

9. Hoisting

JavaScript “hoists” variable and function declarations to the top.

 
console.log(x); // undefined var x = 5;

10. Let, Const, and Var

  • var: function-scoped, can be redeclared.

  • let: block-scoped, can be updated but not redeclared.

  • const: block-scoped, cannot be reassigned.

11. Event Bubbling and Capturing

Events travel:

  • Bubbling → child → parent

  • Capturing → parent → child

12. Event Loop

JavaScript is single-threaded. The event loop manages the stack (synchronous tasks) and queue (asynchronous tasks) so nothing gets stuck.

13. this Keyword

this refers to the object that’s calling the function.

  • In a method → object itself

  • In global scope → window (in browsers)

14. Function Declaration

 
function sayHi() { console.log("Hi!"); }

15. Function Expression

 
const sayHi = function() { console.log("Hi!"); };

16. Prototypal Inheritance

Objects in JavaScript can inherit properties from other objects using prototypes.

17. Promise

Used for handling async tasks.

 
let promise = new Promise((resolve, reject) => { resolve("Done!"); });

18. "use strict"

Makes JavaScript more strict about errors. Prevents silent mistakes.

19. Map Object

Stores key-value pairs. Keys can be objects, not just strings.

20. Set Object

Stores unique values. No duplicates allowed.

21. Async / Await

Syntactic sugar for promises. Looks synchronous, but isn’t.

22. Higher-Order Functions

Functions that take other functions as arguments or return them. Example: map(), filter().

23. Currying

Breaking a function with multiple arguments into multiple functions with one argument each.

24. Memoization

Caching function results so repeated calls are faster.

25. Array Methods

  • map() → transform each element

  • filter() → keep only matching elements

  • reduce() → combine values

  • forEach() → loop through

  • find() → first matching element

  • some() → checks if at least one matches

  • every() → checks if all match

  • sort() → sorts values