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Browser JavaScript Runner & Console

Browser JavaScript Runner & Console

Run JavaScript in a separate Web Worker and inspect console output, the last expression, errors, and execution time.

Focus mode
JavaScript
⌘/Ctrl+Enterto run
Console
Run the code to display console output here

Key features

  • Runs JavaScript from a CodeMirror editor in a separate Web Worker.
  • Formats console log, info, warn, error, debug, and the last expression's completion value.
  • Handles synchronous or Promise results and reports runtime errors and measured duration.
  • At launch, the host schedules a timer to attempt Worker termination after three seconds and also offers a manual Stop action.
  • Defensively blocks common network globals, dynamic import, eval, and Function-family constructors.

How to use

  1. Write JavaScript in the editor.
  2. Press Run or use Cmd/Ctrl+Enter.
  3. Inspect console entries, the last expression, errors, and execution time.

Examples

  • Check an algorithm snippet

    Run a function or array transformation and inspect both console output and the final expression's value.

  • Debug a short data transformation

    Revise and rerun JavaScript without a project while reviewing error stacks and execution time.

Privacy

Your code is kept in that browser's local storage and executes in a separate Web Worker.

Requirements

Network and dynamic-code guards plus the three-second cutoff are defense layers, not guarantees of complete isolation or control over every behavior. Do not run untrusted code or secrets.

Common questions

Can I use fetch or dynamic import?
No. The execution environment is configured to block common network paths and dynamic imports.
What happens if my code does not finish?
Each run uses a separate Web Worker, and that worker is terminated after three seconds.

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