What is TypeScript?

The types exist only while you are writing and building. The compiler checks that the shapes you claimed are consistent, then strips every annotation away — what runs in the browser is plain JavaScript. The value is in the checking and in the tooling it enables: accurate autocomplete, safe renaming, and an editor that can tell you what a function actually returns.

It is straightforward to get no benefit from it. A codebase that reaches for a permissive escape hatch whenever a type is inconvenient has all the ceremony and none of the safety. The types that pay for themselves are the ones on the boundaries — API responses, form input, anything crossing between systems — because that is where the data you assumed and the data you received actually diverge.