About the JavaScript Compiler

The JavaScript Compiler runs code in a fresh, isolated Web Worker with network access removed, so you can try snippets, algorithms and interview questions safely. console output, errors and timing appear instantly. Use it as an online JavaScript compiler or a quick JS runner: write code, click Run and execute JS online without Node.js or a browser console.

Your code runs in an isolated Web Worker in your browser, so it never leaves your device.

How to use the JavaScript Compiler

1. Write or paste JavaScript (or start from the sample).
2. Add input in the stdin box if your code reads it with prompt() or readline().
3. Click Run and read the console output, errors and execution time.

Example

const name = readline() ?? 'world';
console.log(`Hello, ${name}!`);

const scores = [72, 95, 88];
const average = scores.reduce((a, b) => a + b, 0) / scores.length;
console.log('Average:', average.toFixed(1));

Hello, Ada!
Average: 85.0

Common mistakes

• import and require. Code runs as a single script without a module system, so import statements and require() are not available. Paste helper functions into the editor instead.
• document and window. Code runs in a Web Worker, which has no page. DOM APIs such as document.querySelector are undefined; use the HTML Viewer for DOM experiments.
• Undeclared variables. Code runs in strict mode, so assigning to a name without let, const or var throws a ReferenceError instead of creating a global.

Works well with the JavaScript Compiler

• TypeScript Compiler: add types and have them checked before running.
• JavaScript Formatter: format the code before sharing it.
• Regex Tester: try out the regular expressions your code uses.

Frequently asked questions

Guides and tutorials

• Programming Languages

  The JavaScript Event Loop: Promises, setTimeout and Microtasks in Practice

  Why setTimeout(fn, 0) runs after promises, how await pauses, why busy loops delay timers, and how to run async work in sequence or parallel, with tested output.

  October 15, 2026 · 6 min read

• Code Quality & Workflow

  Regular Expressions for Developers: 12 Patterns You'll Actually Use

  Tested regular expressions for emails, dates, UUIDs, versions and logs, plus groups, lookaheads, greedy vs lazy matching and catastrophic backtracking.

  October 15, 2026 · 6 min read

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