What is TypeScript?
TypeScript is JavaScript plus types. A type says what kind of value something holds, such as a number or some text. The compiler checks your code before it runs and catches mistakes, like adding a number to the word "apple", while you are still writing.
When you press Run on this site, your code is type-checked first. If it is fine, the types are removed and the JavaScript runs. (The first Run downloads the TypeScript compiler, so it takes a moment.)
The : string and : number parts are type annotations. Now break it on purpose: change 2026 to "soon" and Run again. TypeScript refuses and tells you why.
When does TypeScript check your types?
Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.
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