Playground

A blank page with the runner attached, for trying an exercise before you read the solution.

This runs JavaScript. The TypeScript solutions on the other pages were compiled when this site was built, and what you press Run on is the JavaScript they compiled to. There is no TypeScript compiler in a browser, so type annotations typed in here will be a syntax error. To practise TypeScript properly, work in a real file:

npm install --save-dev typescript
node scratch.ts             # strips the types and runs it
npx tsc --noEmit scratch.ts # the one that finds your mistakes