JavaScript Online Runner
Write JavaScript, choose Run, and inspect console.log, info, warn and error output in a separate worker. Shared links load code for review without executing it.
Up to 100,000 UTF-16 characters. Explicit Run only; shared links never run automatically.
JavaScript worker context, with top-level await. No page DOM, Node.js, network, storage or external libraries. Await asynchronous work before the script returns.
Console: at most 100 entries and 50,000 displayed characters; values are shortened. Each run stops after five seconds. This is not a hard memory quota; large allocations can still put pressure on your browser.
Execution stays in this browser. Copy share URL puts UTF-8 base64 code in the URL fragment. Base64 is readable encoding, not secrecy; anyone with the link can read the code.
A QUICK WALKTHROUGH
How to use this tool
- Enter up to 100,000 UTF-16 characters of JavaScript.
- Choose Run and review up to 100 console entries and 50,000 displayed characters.
- Copy the console, stop or reset the run, or explicitly copy a share URL after checking that the code contains no secrets.
A JavaScript worker environment
Code runs in an AsyncFunction inside a dedicated worker owned by an opaque-origin sandboxed iframe. This is not a full browser page or Node.js: page DOM, network, storage and external libraries are unavailable. Await asynchronous work before the script returns; completion stops the worker and cancels pending tasks.
Bounded runs and output
A five-second deadline includes preparation and compilation. Completion, errors, clearing, resetting or editing stop the owned worker. Console values are shortened and displayed as text. There is no hard memory quota; large allocations may still put pressure on browser resources. Unsupported isolation has no unsafe fallback.
Share deliberately
Execution stays local. Copy share URL explicitly encodes the current code as UTF-8 base64 in the URL fragment, limited to 300,000 encoded characters. Base64 is not encryption: anyone with the link can read the code. Loading a link never runs it automatically.
GOOD TO KNOW
Common questions
Can this run DOM or Node.js code?
No. It provides a restricted JavaScript worker context, rather than a page DOM, Node.js, network access, storage or package installation.
What happens to an infinite loop?
The trusted owner stops its dedicated worker at the five-second deadline. This is a time limit, not a hard memory quota; resource pressure remains possible.
Does a shared link hide my code or run automatically?
No. A link contains readable base64-encoded code and loads it only into the editor. Review it, then choose Run explicitly. Never put secrets in a shared link.