TypeScript at scale

Core details

TypeScript is JavaScript + static types erased at emit—runtime is still JS semantics. Structural typing: shapes matter; nominal brands need tricks.

Inference: let the compiler infer locals; annotate exports and public APIs for stable contracts.

Patterns: discriminated unions for state machines; satisfies to keep literal types; avoid any—prefer unknown + narrowing.

Understanding

interface vs type: merge declaration with interface; unions/maps often need type. Teams pick one consistency rule.

Build modes: project references for monorepo compile graphs; path aliases need runtime resolution strategy (bundler).

Senior understanding

Types don’t catch all bugs: IO boundaries still need validation (zod/io-ts). Generic abuse hurts readability—balance reuse vs clarity.

Diagram

.ts source → tsc (types erased) → .js → Node / browser
                │
                └── types as documentation + CI gate

See also

• /languages/typescript
• /languages/javascript