language-typescriptlisted

# TypeScript / JavaScript ## Strict mode or nothing Enable in `tsconfig.json`: ```json { "compilerOptions": { "strict": true, "noUncheckedIndexedAccess": true, "noImplicitOverride": true, "exactOptionalPropertyTypes": true } } ``` Without these, types lie. ## Prefer `unknown` over `any` `any` turns off the type system. `unknown` forces narrowing. ```ts function parse(input: unknown): User { if (!isUser(input)) throw new Error("invalid user"); return input; } ``` ## Discriminated unions over enums Enums have runtime quirks and poor exhaustiveness. Use string-literal unions with a discriminant. ```ts type Event = | { kind: "click"; x: number; y: number } | { kind: "submit"; form: FormData }; function handle(e: Event) { switch (e.kind) { case "click": return trackClick(e.x, e.y); case "submit": return submit(e.form); default: { const _exhaustive: never = e; return _exhaustive; } } } ``` ## Branded types for domain primitives Stop `UserId` from being passed where an `OrderId` is expected. ```ts type Brand<K, T> = K & { readonly __brand: T }; type UserId = Brand<string, "UserId">; type OrderId = Brand<string, "OrderId">; const makeUserId = (s: string): UserId => s as UserId; ``` ## Async patterns - `await` everything that returns a promise. Floating promises silently swallow errors. - Enable `@typescript-eslint/no-floating-promises`. - `Promise.all` for all-or-fail; `Promise.allSettled` when partial failure is accep