TypeScript reference · TypeScript Course

Use this index when you already know the question. For the complete sequence, return to the learning path. For an error message, search the course.

Runtime versus static contracts

Question	Start here
Why doesn't a type annotation validate JSON?	Runtime validation
What is unknown vs any?	any, unknown, never, void
Why can a readonly value change?	Objects and aliasing
Why does this compile and still fail?	Runtime safety
Is a brand an authorization check?	Advanced type design

Inference and modeling

Concept	Lesson
Literal widening, const versus let	Values and types
Control-flow analysis, guards, truthiness	Narrowing
Structural typing, excess property checking	Structural typing
Interface extension, intersections, merging	Interfaces and aliases
Assertions, as const, satisfies	Assertions and satisfies
Generic constraints and inference	Generics
keyof, typeof, T[K]	Indexed access
Partial, Pick, Omit, key remapping	Mapped types
infer, distributive conditionals	Conditional types
String event names	Template literal types
Discriminated unions, never, state machines	Exhaustiveness
Contravariance, covariance, method bivariance	Variance

Applications and libraries

Concept	Lesson
target, lib, strict, exactOptionalPropertyTypes	tsconfig
NodeNext, moduleResolution, ESM/CJS, package exports	Modules
Promises, AbortController, async iteration	Async
fetch, status, retries, pagination	HTTP clients
Tests, inference tests, expected failures	Testing
ESLint, Prettier, compiler responsibilities	Static analysis
Process signals, configuration, graceful shutdown	Node application
Overloads, public inference	Library design
.d.ts, ambient declarations, DefinitelyTyped	Declaration files
npm pack, types, ESM delivery	Publishing
Workspaces, project references	Large repositories
Compiler timings, native parallelism	Performance
allowJs, checkJs, JSDoc	Migration
Prototypes, private fields, generics in classes	Advanced design
Domain boundaries, adapters, DTOs	Architecture
Parser, binder, checker, emit, LSP	Compiler tooling
CI, upgrades, semver, release evidence	Shipping

Error-oriented index

Diagnostic family	What to inspect
TS2322: not assignable	Required shape, possible absence, function direction; structural typing
TS2345: argument not assignable	Parameter contract and narrowing; generics
TS18046: unknown value	Evidence at the boundary; runtime validation
TS2532 / TS18047: possibly undefined or null	Presence policy; errors and null
TS2353: extra property	Fresh literal versus wider object; structural typing
TS2375: exact optional properties	Missing versus explicit undefined; tsconfig
TS2540: readonly write	Mutation ownership; objects
TS1361: type-only import used as value	Erased binding versus runtime import; modules
TS2578: unused expect-error	Stale or ineffective negative test; testing

Exact diagnostics depend on source and the pinned compiler. The numbered lessons show real captured diagnostics, including locations and alternatives, rather than a universal error-text promise.