JavaScript Online Runner

About this tool

JavaScript Online Runner executes JavaScript code in a sandboxed iframe with no external dependencies. Each run builds a hidden iframe whose srcdoc holds a small console-override shim followed by your code, with the sandbox flag 'allow-scripts' only, and output is relayed back with postMessage. console.log, console.error, console.warn and console.info calls are captured; errors and warnings get ✖ and ⚠ markers and colours. Objects and arrays are printed with JSON.stringify(value, null, 2). Uncaught errors, including syntax errors, are caught by a window error listener. Each run is stopped after 5 seconds, so later timers or promises do not print. Because the tool uses no CDN or bundler, it loads instantly. Copy share link encodes your snippet as base64 in the URL fragment, so you can share runnable code without any backend. This tool is ideal for quickly testing JavaScript expressions, DOM-free algorithms, short async flows and JSON transformations.

Why use it

• Zero dependencies — no CDN, no bundler, no npm install, starts instantly on the slowest connection.

• Sandboxed execution — code runs isolated from the parent page

• Captures the four main console methods with colour-coded output

• Shareable snippets via encoded URL fragment

• Works entirely in the browser — no server needed

How to use

1. Type or paste JavaScript into the editor.
2. Click Run JavaScript or press Ctrl/⌘ + Enter — output appears in the Output panel.
3. Use console.log(), console.error(), console.warn(), and console.info() to inspect values.
4. Click Copy share link to copy a URL that contains your snippet.
5. Click Clear to wipe the console and stop a running snippet.

When it helps

• Quickly testing a JavaScript expression, regex, or one-line algorithm
• Demonstrating a code snippet to someone without a shared editor
• Debugging logic before adding it to a larger project
• Teaching JavaScript to students with an instant feedback loop
• Validating short async/await flows or Promise chains

Examples

const nums = [1, 2, 3, 4, 5, 6];
const evenSquares = nums.filter(n => n % 2 === 0).map(n => n * n);
console.log(evenSquares);

[
  4,
  16,
  36
]

const wait = ms => new Promise(r => setTimeout(r, ms));
(async () => {
  console.log('start');
  await wait(500);
  console.log('after 500ms');
})();

start
after 500ms

const user = { name: 'Ada', roles: ['admin', 'editor'], meta: { last: '2026-01-01' } };
const { name, roles: [primary] } = user;
console.log(JSON.stringify({ name, primary }, null, 2));

{
  "name": "Ada",
  "primary": "admin"
}

Tips

• Wrap top-level await calls in an async IIFE: (async () => { /* code */ })() — top-level await is not supported in script-mode iframes.
• console.log already pretty-prints objects and arrays with JSON.stringify(value, null, 2); use console.log(String(arr)) for a one-line 4,16,36 style output.
• Each Run resets the iframe — use it to clear stuck timers or intervals from a previous run.
• URL-shared snippets travel in the fragment (#...) so they never hit the server, even when shared via chat or email.
• Combine setTimeout with console.log to trace event-loop ordering and microtask vs macrotask behaviour; keep delays under 5 seconds.
• Use performance.now() to benchmark small algorithms — the iframe has full access to the standard timing API.

Frequently Asked Questions

Glossary

Sandboxed iframe

An HTML iframe with the sandbox attribute that restricts access to the parent page's DOM, cookies, top-level navigation, and form submission.

srcdoc

An iframe attribute that holds the HTML of the frame inline; this tool puts the console shim and your code there, so no network request is needed to load it.

postMessage

A browser API for cross-origin communication between windows or iframes, used here to relay console output from the sandbox back to the runner UI.

REPL

Read-Eval-Print Loop — an interactive evaluator. This tool is a script-mode REPL where each Run evaluates the full editor contents.

ES2020

The 11th edition of the ECMAScript standard, adding optional chaining (?.), nullish coalescing (??), BigInt, and dynamic import among other features.

Microtask vs Macrotask

Promise callbacks run on the microtask queue (immediately after current code), while setTimeout uses the macrotask queue (next event-loop tick).

Unhandled promise rejection

A rejected Promise without a .catch handler. Browsers raise an unhandledrejection event for it; this tool does not listen for that event, so such rejections do not appear in the output.

URL fragment

The portion of a URL after the # character; never sent to the server, making it safe for embedding shareable code without backend storage.