Free Online JavaScript Compiler
JS CodeLab is a free online JavaScript runner and playground for writing, running, testing, and learning JavaScript directly in your browser. Use this lightweight JS sandbox without any installation or account.
Test JavaScript Online
Execute modern JavaScript code instantly. Our secure in-browser execution engine evaluates your code securely without requiring server round-trips, giving you immediate feedback.
JavaScript Console Online
Experience a professional IDE interface powered by Monaco Editor. Enjoy syntax highlighting, intelligent autocompletion, real-time diagnostics, and formatting right in your browser.
Learn JavaScript With Examples
Explore our curated library of interactive examples. From basic variables to async/await, test concepts and see how JavaScript works in real-time.
Popular Examples
Jump right into code with these runnable snippets.
JavaScript Hello World Program
The JavaScript Hello World program is the foundational entry point for learning JavaScript. It uses the standard console.log() API to print text to the debugging console without requiring any external libraries or browser DOM manipulation.
JavaScript Variables (let vs const vs var)
JavaScript variables are named memory containers used to hold and manipulate data values throughout runtime execution. Modern JavaScript uses const for fixed identifier bindings and let for mutable variables, enforcing block scope and eliminating var.
JavaScript Template Literals (String Interpolation)
JavaScript template literals are string literals delimited by backticks (`) that enable embedded expressions, multi-line string authoring, and string interpolation using the ${expression} syntax without cumbersome plus-sign concatenation.
JavaScript if-else Conditional Statements
JavaScript conditional statements execute distinct blocks of code based on whether a specified condition evaluates to truthy or falsy. The if...else syntax provides structured branching control flow across all ECMAScript environments.
JavaScript Function Declaration & Invocation
A JavaScript function declaration defines a reusable, parameterized subroutine using the function keyword. In JavaScript, functions are first-class values that can be assigned to variables, passed as arguments, and returned from other functions.
JavaScript Arrow Functions (ES6 Syntax)
JavaScript arrow functions provide a concise syntax for writing function expressions using the => token. Unlike traditional functions, arrow functions do not bind their own this, arguments, or super, lexically capturing them from their enclosing scope.
Learn JavaScript
Master the fundamentals with interactive guides.
JavaScript Variables Explained: let vs const vs var
A JavaScript variable is a named storage container for holding data values in memory. Modern JavaScript uses const by default for immutable bindings and let for variables that must be reassigned, completely replacing the obsolete function-scoped var keyword.
JavaScript Functions Guide: Declarations, Expressions & Arrow Functions
A JavaScript function is a reusable block of code designed to perform a specific task, accept arguments, and return computed results. JavaScript functions are first-class citizens, meaning they can be assigned to variables, passed as arguments, and returned from other functions.
JavaScript Arrays Guide: Methods, Iteration & Manipulation
A JavaScript array is an ordered list of values that can store multiple data types under a single variable name. Arrays feature zero-based indexing and provide a rich set of built-in functional transformation methods.
JavaScript Promises & Async/Await Guide (with Examples)
A JavaScript Promise represents the eventual completion or failure of an asynchronous operation and its resulting value. Async/await provides synchronous-like syntax for handling Promises without callback hell.
JavaScript Closures Explained (with Real Examples)
A closure is the combination of a function bundled together with references to its surrounding lexical environment. Closures give an inner function access to an outer function's scope even after the outer function has finished executing.
Array map() vs forEach() in JavaScript: Key Differences
The primary difference between map() and forEach() is that map() allocates and returns a new array containing transformed items, whereas forEach() executes a side effect for each element and returns undefined.
What is an online JavaScript compiler?
An online JavaScript compiler (or JS runner) is a web-based tool that allows you to write, edit, and execute JavaScript code directly in your internet browser. Unlike traditional local development environments, JS CodeLab doesn't require you to install Node.js, configure build tools, or set up a local server. You can simply open the website and start coding immediately in a safe JS sandbox.
How do I run JavaScript online?
Testing JavaScript online with JS CodeLab is simple:
- Open the Playground.
- Type your JavaScript code into the editor.
- Click the "Run" button or press Ctrl + Enter.
- View your output immediately in the integrated JavaScript console online.
Privacy and how it works
JS CodeLab is built with a strictly local-first architecture to guarantee your privacy and ensure lightning-fast execution:
- Execution in Web Workers: Your code is executed safely in an isolated background thread (Web Worker) directly within your browser.
- Code Saved in IndexedDB: All your projects, snippets, and settings are saved locally to your browser's IndexedDB.
- Nothing Sent to Servers: Your code is never transmitted, processed, or saved on any external servers. The only network requests made are to download the static assets required to run the editor.
Frequently Asked Questions
Is JS CodeLab free?
Yes, JS CodeLab is a 100% free online JavaScript playground.
Does JS CodeLab require installation?
No installation is required. JS CodeLab runs entirely within your web browser using client-side Web Workers for execution.
Can I test modern JavaScript features?
Absolutely. You can write and test modern ECMAScript features including async/await, arrow functions, and destructuring.