JavaScript Interview Questions

 

JavaScript Interview Guide: Essential Questions & Detailed Answers

Welcome to the ultimate guide to help you crack JavaScript interviews! This page contains well-researched and in-depth answers to common questions asked in front-end developer interviews, especially focused on JavaScript core concepts.

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Table of Contents

1. Introduction to JavaScript
2. Data Types and Variables
3. Functions and Scope
4. Object-Oriented JavaScript
5. Asynchronous JavaScript
6. Advanced Topics (Closures, Hoisting, etc.)
7. Real-World Scenarios
8. Bonus Tips for Interviews

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1. Introduction to JavaScript

JavaScript is a high-level, interpreted scripting language primarily used to create dynamic and interactive effects within web browsers. It’s one of the core technologies of the web, alongside HTML and CSS.

Why do companies ask JS questions in interviews?
Because JavaScript is the backbone of front-end development and is increasingly used for full-stack solutions with Node.js.

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2. Data Types and Variables

Q: What are the different data types in JavaScript?

Answer:
JavaScript has two main categories of data types:

• Primitive Types: string, number, boolean, null, undefined, bigint, symbol.
• Reference Types: object (including arrays, functions, and more complex structures).

Real-world tip: Always check type coercion when comparing values to avoid unexpected bugs (== vs ===).

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Q: Explain the difference between var, let, and const.

Feature	var	let	const
Scope	Function-scoped	Block-scoped	Block-scoped
Re-declaration	Allowed	Not allowed	Not allowed
Hoisting	Yes (initialized as undefined)	Yes (but no initialization)	Yes (but no initialization)
Re-assignment	Yes	Yes	No

Insight: Avoid using var in modern JavaScript. Always prefer const unless you expect to reassign a variable, then use let.

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3. Functions and Scope

Q: What is the difference between function declaration and function expression?

• Function Declaration:

  function greet() {
      console.log("Hello!");
  }
  

  Hoisted to the top of the scope.

• Function Expression:

  const greet = function() {
      console.log("Hello!");
  }
  

  Not hoisted, treated like a regular variable.

Pro Tip: Use function expressions inside callbacks or when using higher-order functions.

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4. Object-Oriented JavaScript

Q: Explain prototypal inheritance.

Answer:
Every JavaScript object has a hidden property [[Prototype]] that refers to another object. Objects inherit properties and methods from their prototype chain.

const animal = { eats: true };
const dog = Object.create(animal);
console.log(dog.eats); // true

Best Practice: Prefer class syntax when working on large-scale projects to improve readability.

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5. Asynchronous JavaScript

Q: Explain callback, promise, and async/await.

• Callback: A function passed as an argument to another function.
• Promise: An object representing the eventual completion or failure of an asynchronous operation.
• Async/Await: Syntactic sugar over promises to write asynchronous code that looks synchronous.

Example:

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return data;
}

Real-world insight: Always handle errors using try...catch with async/await to avoid unhandled promise rejections.

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6. Advanced Topics

Q: What is a closure?

Answer:
A closure is a function that retains access to its lexical scope even when executed outside of that scope.

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

Use Case: Closures are widely used in data privacy and currying.

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Q: What is hoisting?

Answer:
Hoisting moves declarations to the top of the scope during the compile phase. However, only the declaration is hoisted, not the initialization.

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

Interview tip: let and const are also hoisted but stay in the Temporal Dead Zone (TDZ) until initialization.

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7. Real-World Scenario Questions

Q: Explain a situation where this binding confused you.

Answer:
In event handlers, this inside a regular function refers to the DOM element, but inside an arrow function, it refers to the lexical scope.

button.addEventListener('click', function() {
  console.log(this); // <button> element
});

button.addEventListener('click', () => {
  console.log(this); // window or enclosing object
});

Solution: Prefer arrow functions if you want this to refer to the parent scope, especially inside classes or modules.

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8. Bonus Tips for Interviews

• Understand core concepts like event loop, async/await, and closures deeply.
• Write clean, modular code during whiteboarding or take-home assignments.
• Explain your thought process clearly to the interviewer.
• Be ready with real-world examples (e.g., "In my last project, I used closures for creating private variables in a module pattern.").