Javascript interview questions are one of the most searched topics by developers preparing for frontend, backend, and full-stack roles. Javascript is the backbone of modern web development, powering interactive websites, web applications, mobile, and even server-side systems.
Many companies test JavaScript knowledge not only to check syntax understanding but also to evaluate problem-solving skills, logical thinking, and real-world application concepts. This article covers important javascript interview questions with clear explanations, making it suitable for freshers, mid-level developers, and experienced professionals.
Why are the JavaScript Interview Questions Important ?
JavaScript is the best logical language, which many interviewers focus on. Some of the important topics in JavaScript that the interviewer focuses on are:
- Core languages concepts
- How javascript works internally
- Performance and optimization
- Real-world coding scenarios
Most Interviewers assess whether you understand how javascript behaves behind the scenes, not just how to write code.
Basic JavaScript Interview Questions
Q1. What is JavaScript?
Answer 1: JavaScript is a high-level, interpreted programming language used to create dynamic and interactive web pages. it runs in the browser as well as on servers using environments like Node.js.
Key Features of JavaScript:
- Lightweight and fast
- Event-driven
- Supports object-oriented programming
- Cross-platform compatibility
Q2. What are the different data types in javascript ?
Answer 2: Javascript has two main categories of data types, which are primitive data types and non primitive data types.
Primitive data types consist of:
- String
- Number
- Boolean
- Undefined
- Null
- Symbol
- BigInt
Non-primitive Data types:
- Arrays
- Functions
- Dates
- Linkedlist
Q3: Differences between Var, Let, and Const
Answer 3: It is one of the most common and important JavaScript interview questions.
- Var has function scope and can be redeclared
- let used to block scope and cannot be redeclared
- Const has block scope and cannot reassigned
- Modern Javascript prefers let and const due to better scope control.
Intermediate Javascript interview questions
Q1. What is hoisting in JavaScript?
Answer 1: Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their scope during compilation.
- Variables declared with var are hoisted but initialized as undefined
- Let and const are hoisted but stay in the temporal dead zone
- Function declarations are fully hoisted.
Q2. What is closure?
Answer 2: A closure is something that is created when a function remembers variables from its external scope, even after the outer function has finished executing.
Closures are used in:
- Data privacy
- Event Handlers
- Callbacks
- Functional Programming
Closures are frequently asked about in javascript interviews because they test deep conceptual understanding.
Q3. Explain this keyword in JavaScript
Answer 3: this keyword refers to the object that is executing the current function.
Its value depends on:
- Globe context
- Object method
- Constructor function
- Arrow functions
Understanding these functions is critical for writing a clean and predictable JavaScript code.
Q4. What is the difference Between == and ===?
- == checks value equality and performs type conversion.
- === checks both value and type (strict equality)
- Using === is best to avoid unexpected bugs.
Advanced JavaScript Interview Questions
Q1. What is the JavaScript event loop?
Answer 1: The event loop is responsible for handling asynchronous operations in javaScript.
It works with the following:
- Call stack
- Callback Queue
- Microtask queue
Q2. What are Event bubbling and event capturing?
Event propagation has three phases:
- Capturing phase
- Target phase
- Bubbling phase
Event bubbling means events propagate from the target element to the parent elements; event capturing means events propagate from parent to child.
Q3. Difference between Synchronous and Asynchronous Javascript
Synchronous Javascript:
- Executes tasks in code line by line
- Blocks Execution
Asynchronous JavaScript:
- Executes tasks in the background
- Uses callbacks, promises, and async/await
- Most real-world JavaScript applications rely heavily on asynchronous programming
JavaScript DOM interview Questions
Q1. What is DOM?
Answer 1: DOM stands For Document Object Model. It represents the HTML documents as a tree structure where JavaScript can manipulate elements, attributes, and style dynamically.
Q2. Difference between getElementById and querySelector
- getElementById selects an element by ID
- querySelector selects elements using CSS selectors
JavaScript Performance & Best Practices
Q1. How can you Improve JavaScript performance ?
- Minimize DOM manipulation
- Use Event delegation
- Avoid global Variables
- Optimize Loops
- Use async Loading
- Performance optimization questions are common in senior-level interviews
Q2. What are debouncing and throttling?
- Debouncing limits function execution after a delay
- Throttling limits execution to a fixed time interval
Used in search boxes, scroll events, and resize events.
