Bench reads JavaScript you paste — it highlights the code, lists its structure (functions, classes, and variables), runs it in an isolated background worker, and shows the result and anything you log. If there's a syntax error, it's pinpointed by line and highlighted for you. Everything runs on your device; your code is never uploaded. This runs plain JavaScript — the page's DOM and window aren't available.

Run as I type

Code & structure

Structure

Output

Syntax checking and the structure outline use the Acorn parser, bundled with the page so it works offline. Your code runs in a Web Worker with a short time limit, so an accidental infinite loop is stopped instead of freezing the tab. Because it's a worker, there's no document, window, or alert — use return values and console.log to see output. Nothing is uploaded.