AnyTool
Your files never leave your device. All processing happens locally in your browser.

How do I run JavaScript online without installing anything?

Type JavaScript into AnyTool's playground and it runs as you type in a sandboxed iframe, streaming console.log, warn, and error output with line numbers right below the CodeMirror editor. async/await works, and you can switch on TypeScript or Prettier — all in your browser, so your code never leaves your device.

  • CodeMirror editor with run-as-you-type — no Run button needed
  • Leveled live console (log, warn, error, info, debug) with runtime error line numbers
  • async/await, optional TypeScript transpile, and one-click Prettier formatting
  • Examples library and share-via-URL (state encoded in the link, no backend)
  • 100% client-side — code runs in a sandboxed iframe and never leaves your browser, free, no signup

What is

JavaScript Playground

A JavaScript playground is an online editor where you write JavaScript and see it run instantly with captured console output. AnyTool's runs code in a sandboxed browser iframe — no server execution — with run-as-you-type, error line numbers, and optional TypeScript and Prettier.

Developer Tools

Related terms

JS REPLOnline JavaScript EditorCode SandboxCodeMirrorTypeScript

How to Run JavaScript in the Browser

Write, run, format, and share JavaScript entirely in your browser with a live console.

10 secWeb browser
  1. 1

    Write code

    Type JavaScript into the CodeMirror editor, or load one of the example snippets.

  2. 2

    See it run

    Code runs as you type in a sandboxed iframe and streams console output with error line numbers below.

  3. 3

    Format or share

    Optionally toggle TypeScript or Prettier, then copy the result or share via URL.

Result: Live console output from your JavaScript

Frequently Asked Questions

Yes — it is completely free with no signup, no account, and no limits.

The JavaScript playground is 100% free with no account, subscription, or paid tier. You can write, run, format, and share as much code as you want — unlike PlayCode or RunJS, which gate features behind a Pro account or a paid desktop app.

No — your code runs in a sandboxed iframe on your device and is never uploaded to any server.

Your code is executed in a sandboxed iframe (allow-scripts only, no same-origin) right on your machine, so it cannot read the page, cookies, or local storage, and it is never POSTed to a server. AnyTool is the only playground in its field that both runs and shares code with zero backend, so it works offline and keeps private code private.

No — execution is fully client-side; there is no server-side runtime.

Unlike OneCompiler, Programiz, Replit, or StackBlitz, which execute code on a server or in a cloud VM, AnyTool evaluates your JavaScript locally inside a sandboxed iframe. Console output is captured via postMessage and streamed back live, so nothing is sent to a backend.

Yes — top-level async/await works, and you can toggle TypeScript and Prettier formatting.

Your code is wrapped in an async IIFE so top-level await, Promises, and timers resolve and stream into the console as they fire. You can switch on TypeScript to transpile in the browser before running, and use one-click Prettier formatting — both use already-bundled libraries, so it all stays offline.

Detailed Explanation

Methodology

How the JavaScript Playground Runs Code

AnyTool runs user JavaScript inside a freshly created sandboxed iframe (sandbox="allow-scripts" only, with no same-origin) so the code cannot read the parent page, cookies, or local storage. A bootstrap overrides console.log/warn/error/info/debug and wraps the source in an async IIFE, then posts each result back to the editor via postMessage, so the leveled console streams output live and supports top-level await, Promises, and timers. Execution runs as you type with debounced auto-run, runtime errors are mapped back to source line and column numbers, and optional TypeScript transpile (ts.transpileModule) and Prettier formatting use bundled libraries. Nothing is sent to a server and there is no runtime CDN.

  • Code runs in a sandboxed allow-scripts iframe — no server execution
  • Leveled live console with pretty-printed objects and error line numbers
  • Run-as-you-type with debounced auto-run; async/await via an async IIFE
  • Optional in-browser TypeScript transpile and one-click Prettier formatting
  • Examples library and share-via-URL with state encoded in the link, no backend
Privacy & Security

Privacy and Offline Use

Editing, execution, console capture, formatting, and sharing all happen in the browser with no runtime CDN, so your code never leaves the device — unlike OneCompiler, Programiz, Replit, or StackBlitz, which run code on a server or cloud VM, and unlike server-share tools that need a backend round-trip to save a snippet. AnyTool encodes editor state into the URL hash, so sharing also needs no backend. It works offline after first load and has no signup or paid tier, making it the only playground in its field that both runs and shares code fully locally.

JavaScript playground: AnyTool vs typical online playgrounds
CapabilityAnyToolTypical playgrounds
Execution locationIn-browser sandboxed iframeOften server or cloud VM
Run-as-you-typeYes (debounced auto-run)Often click Run
Share without backendYes (URL-encoded state)Usually server-side link
TypeScript + PrettierBuilt in, in-browserOften paid or absent
Signup / paid tierNoneOften Pro account or app funnel

Capabilities reflect the live AnyTool tool; competitor behavior varies by service.