JS Runner

How to use JS Runner

1. Pick JavaScript or TypeScript, then type or paste code into the editor, load one with Example, or use Open file. Tab indents; press Esc then Tab to move focus out of the editor.
2. Press Run or Ctrl/⌘ + Enter. Output from console.log, console.table, console.group and the other console methods appears in the Console below.
3. Choose a Time limit (5, 10, 30 or 60 s). Press Stop at any time, for example to end an infinite loop or a setInterval.
4. For TypeScript, tick Show compiled JS to see the JavaScript that actually runs.
5. Turn on Timestamps to see when each entry was logged, and use Copy output, Send to… or Clear output on the console.

How it works

Each run starts a fresh Web Worker, created from your code, and terminates it when the run is over, so nothing carries over between runs and the worker can’t read or change the page. Your code is wrapped in an async function, which is why top-level await works. A small runtime replaces console so output is captured and pretty-printed, and keeps track of timers and fetch calls so the run only counts as finished when your code and everything it scheduled are done.

TypeScript is turned into JavaScript with Sucrase, which strips type annotations without type-checking and keeps every line where it was, so error line numbers match your code. Errors and stack traces are mapped back to lines in the editor. Sucrase is downloaded from this site the first time you run TypeScript.

The site’s Content Security Policy also applies inside the worker: requests to other servers are blocked, and eval and new Function don’t work.

Limits

• Runs as a plain script in a worker: there is no DOM, so document, window and alert don’t exist.
• Static import and export statements aren’t supported, and npm packages aren’t available; paste the code you need instead.
• Network requests to other sites are blocked, and eval and new Function are blocked.
• TypeScript types are removed but not checked, so type errors don’t stop the code from running; only syntax errors do.
• Output stops after 2,000 console entries or 1 MB of text, and the run is then stopped. Objects are printed 4 levels deep and with up to 100 items per collection; console.table shows the first 1,000 rows.

Privacy

Your code runs only in your browser, in an isolated worker, and is never uploaded. The editor contents, language and time limit are saved in this browser’s local storage so they are still there when you come back. Code sent here from another tool or a Learn page with Send to… arrives through this tab’s session storage and is removed as soon as it is read; the same applies to console output you send on. This tool doesn’t offer share links.

Frequently asked questions

Why does document or window say it is not defined?

Code runs in a Web Worker, which has no page to work with, so DOM APIs don’t exist there. Use the console methods to see results. Worker APIs such as setTimeout, Promise, crypto and TextEncoder are available.

Can I fetch data from an API?

Not from other sites. The site’s security policy blocks requests to any server except this one, so a fetch to an external URL fails with a network error. Paste the data into your code instead.

Does it type-check my TypeScript?

No. Types are stripped with Sucrase without checking them, so the code runs even if it has type errors. Syntax errors are reported with their line and column before anything runs.

What happens if my code has an infinite loop?

The page stays responsive because the code runs in a separate worker. Press Stop, or wait for the time limit, and the worker is terminated.

When does a run finish?

When your code has finished and no timers or fetch calls it started are still pending. A setInterval that is never cleared keeps the run going until you press Stop or the time limit is reached.

Why can’t I use import?

Code runs as a classic script, not a module, and there is no network access to download packages. Static import and export statements are detected before running and explained instead.