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Intro to RunJS

RunJS is a desktop app for rapidly prototyping JavaScript and TypeScript. Write code on the left and see the results on the right. There's nothing to set up and nothing to build.

RunJS demo

Use cases​

  • Experimenting with an API or library: pull in an npm package and explore its surface without spinning up a project.
  • Learning a language feature: try new ECMAScript or TypeScript syntax and see what it evaluates to.
  • Prototyping logic: work out a tricky function in isolation before pasting it into a real codebase.
  • Quick data transforms: paste in JSON, CSV, or a log dump and reshape it in a few lines.
  • Hitting API endpoints: fire fetch requests at a REST or GraphQL API and inspect the response inline.
  • Prototyping UI: sketch a React component, an SVG, or a canvas animation and see it render in the web view.
  • Browser API experiments: play with WebGL or the Web Audio API without setting up an HTML page.
  • Debugging: drop logpoints or magic comments into a snippet to inspect intermediate values without console.log.
  • One-off scripts: run a calculation, query a local file, or kick off a small Node task without committing anything to disk.