TypeScript / Node.js — Basics

Runtime

• V8 + libuv + Node bindings. JS single-threaded, I/O via libuv thread pool (default 4, UV_THREADPOOL_SIZE).
• libuv uses epoll / kqueue / IOCP for network. Thread pool for file + DNS.

Event loop phases

1. timers — setTimeout/setInterval
2. pending callbacks — deferred I/O
3. idle/prepare — internal
4. poll — fetch I/O events
5. check — setImmediate
6. close — socket.on('close')

Microtasks (process.nextTick, Promise .then) drain between phases. nextTick before Promises. Recursive nextTick starves I/O.

Streams

4 types: Readable, Writable, Duplex, Transform. Always use pipeline() (handles errors). Modes: flowing vs paused.

TypeScript essentials

• Structural typing (shape match).
• any disables checks. unknown requires narrowing. never = impossible.
• type = unions/intersections/mapped/conditional. interface = extensible via merging.
• Narrow via typeof, instanceof, in, type guards (x is Foo), discriminated unions.
• Utility: Partial, Required, Readonly, Pick, Omit, Record, Exclude, Extract, ReturnType, Parameters, Awaited.
• Generics with extends constraints. Conditional T extends U ? X : Y. infer extracts.
• Mapped: { [K in keyof T]: ... }.
• tsconfig: strict, noUncheckedIndexedAccess, target, module, moduleResolution: NodeNext.

Async

• async returns Promise. await suspends.
• Promise.all (fail-fast), allSettled, race, any.
• Don’t await in loop for independent items — Promise.all(arr.map(...)).
• Unhandled rejection crashes Node 16+.

Modules

• CJS (require) vs ESM (import). ESM async, no __dirname default.
• package.json "type": "module" switches default.

Tooling

• npm / pnpm / yarn. npm ci uses lockfile exact.
• Semver: ^1.2.3 minor+patch, ~1.2.3 patch only.