TypeScript

TypeScript adds static typing, tooling, and language-level ergonomics on top of modern JavaScript.

Key Concepts

• Structural typing: compatibility determined by shape, not declarations.
• Gradual typing: opt-in annotations with inference when omitted.
• Declaration merging allows multiple definitions to extend a symbol.
• Type erasure: emitted JavaScript matches configured target without TS constructs.

Important Syntax

• Interfaces and type aliases describe object shapes.
• Generics (`function wrap(value: T): Box { ... }`).
• Union and intersection types for expressive APIs.
• Utility types: Partial, Required, Pick, Omit, ReturnType, Awaited.
• Decorators (stage 3) for metadata-driven frameworks.
• enum, const enum, and literal types for finite sets.

Type System Power Tools

• Conditional types: `T extends U ? X : Y`.
• Mapped types iterate over keys and transform them.
• Infer keyword extracts types inside conditional branches.
• Template literal types compose string unions.
• Satisfies operator validates shape without widening values.

Operators & JS Interop

• All ECMAScript operators including optional chaining `?.`, nullish coalescing `??`, logical assignment `??=`.
• Readonly modifier ensures immutable fields and tuples.
• Definite assignment assertions (!) silence checks when safe.
• Non-null assertion `value!` tells the compiler you ensured defined.

Tooling

• tsconfig.json controls module targets, strict mode, path mapping.
• tsc, swc, esbuild, and Vite compile TypeScript for browsers or Node.
• ESLint with @typescript-eslint enforces style and correctness.
• Type acquisition (Automatic Type Acquisition) fetches DefinitelyTyped packages.

Best Practices

• Enable `strict` (or at least `strictNullChecks`, `noImplicitAny`).
• Prefer type-only imports (`import type { Foo } from "./foo"`).
• Express API contracts via interfaces instead of `any`/`unknown`.
• Leverage discriminated unions for state machines and reducers.

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