TypeScript

How TypeScript decides what type your value has — without you saying so.

Same job 80% of the time. The 20% where the answer is forced.

Type parameters, constraints, defaults, inference at the call site.

Tag, narrow, exhaust — the safest pattern for variant data.

Partial, Pick, Omit, Record, ReturnType — the tools you should reach for first.

typeof, instanceof, `in`, predicates, assertions — and which to reach for.

extends, infer, distribution, mapped key remapping — TypeScript at type level.

ESM vs CJS, .d.ts files, ambient declarations, module augmentation, types-only imports.

The flags that turn TypeScript from suggestions into enforcement.