🚀 Ultimate Guide: JavaScript Interview Questions and Answers (2025 Edition) - Part 3

In this third part of our JavaScript interview series, we will focus on theoretical concepts that are crucial for understanding JavaScript at a deeper level. These questions will help you strengthen your fundamental knowledge and impress interviewers with clear explanations. 📌

Let’s dive in! 🔥

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🟢 JavaScript Theoretical Questions

1. What is the difference between JavaScript and ECMAScript?

ECMAScript (ES) is a standard that defines how JavaScript should work. JavaScript is an implementation of ECMAScript.

• ECMAScript provides rules, guidelines, and updates (e.g., ES6, ES7).
• JavaScript follows these specifications and adds web-specific features (DOM, BOM).

2. What is the difference between compilation and interpretation?

• Compilation: The entire code is converted into machine code before execution (e.g., C, Java).
• Interpretation: Code is executed line by line, as in JavaScript.
• JavaScript is JIT (Just-in-Time) compiled, meaning it is a mix of both.

3. What is the JavaScript Execution Context?

It is an environment where JavaScript code is executed. There are three types:

1. Global Execution Context (GEC): Created by default.
2. Function Execution Context (FEC): Created for each function call.
3. Eval Execution Context: Created inside eval().

4. What are Hoisting and Temporal Dead Zone (TDZ)?

• Hoisting: JavaScript moves variable and function declarations to the top before execution.
• TDZ (Temporal Dead Zone): The time between variable creation and initialization (for let and const).

Example:

console.log(a); // undefined (Hoisted)
var a = 10;

console.log(b); // ReferenceError (TDZ)
let b = 20;

5. What is Scope in JavaScript?

Scope determines the visibility of variables:

• Global Scope: Accessible anywhere in the program.
• Local Scope: Exists within functions.
• Block Scope: Available only within {} (for let and const).
• Lexical Scope: Functions can access variables from their parent scope.

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🟡 Advanced Theoretical Concepts

6. What is the difference between a Polyfill and a Transpiler?

• Polyfill: A piece of code that provides modern functionality in older browsers.
• Transpiler: Converts modern JavaScript (ES6+) into older versions using tools like Babel.

7. What are Higher-Order Functions in JavaScript?

A function that takes another function as an argument or returns a function.

Example:

function multiplier(factor) {
  return function (num) {
    return num * factor;
  };
}
const double = multiplier(2);
console.log(double(5)); // 10

8. What is the difference between a Callback and a Promise?

• Callback: A function passed into another function and executed later.
• Promise: An object representing an asynchronous operation with states (Pending, Fulfilled, Rejected).

9. What are Generator Functions in JavaScript?

A special function that can be paused and resumed using yield and next().

Example:

function* generatorFunc() {
  yield 1;
  yield 2;
  yield 3;
}
const gen = generatorFunc();
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2

10. What is the difference between localStorage, sessionStorage, and cookies?

Storage Type	Lifespan	Size	Sent with Requests?
localStorage	Permanent	5MB	❌ No
sessionStorage	Until tab closes	5MB	❌ No
Cookies	Defined by expiry	4KB	✅ Yes

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🔴 Miscellaneous Theoretical Questions

11. What is the difference between Event Bubbling and Event Capturing?

• Event Bubbling: Events start at the target element and bubble up to the parent elements.
• Event Capturing (Trickling): Events start at the root and go down to the target.

Example:

document.querySelector("button").addEventListener("click", () => {
  console.log("Button clicked");
}, true); // Capturing mode

12. What is the difference between a WeakMap and a Map?

Feature	Map	WeakMap
Keys	Any value	Only objects
Garbage Collection	❌ No	✅ Yes (keys are weakly held)
Iteration	✅ Yes	❌ No

13. What is the difference between a shallow copy and a deep copy?

• Shallow Copy: Copies only references to nested objects.
• Deep Copy: Creates a completely independent clone.

Example:

let obj1 = { a: 1, b: { c: 2 } };
let shallowCopy = { ...obj1 };
let deepCopy = JSON.parse(JSON.stringify(obj1));

14. What is Defer and Async in JavaScript?

• Async: Loads script asynchronously but executes immediately.
• Defer: Loads script asynchronously but executes in order after HTML is parsed.

Example:

<script src="script.js" async></script>
<script src="script.js" defer></script>

15. What is Functional Programming in JavaScript?

A paradigm where functions are first-class citizens and avoid changing state.

• Uses pure functions.
• Avoids side effects.
• Uses higher-order functions.

Example:

const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

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🎯 Final Thoughts

Understanding these JavaScript theoretical concepts is essential for acing interviews. Make sure to practice and apply them in real-world projects! 🚀

💬 Got more questions? Drop them in the comments! 👇

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