TypeScript
The lab runs a real TypeScript compiler in the browser, not a simulation. You see the exact type errors your IDE would show.
Free interactive TypeScript course in 10 lessons: inference, interfaces, unions and narrowing, generics, utility types, compile time vs runtime, any vs unknown. With a real type-checker in your browser.
Free, interactive, no signup: https://www.web-developpeur.com/en/learning/typescript/
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