TypeScript: основы типов и inference — пример кода

// Inference vs annotation — annotate inputs, let outputs infer
function double(n: number) {   // ✅ annotate the parameter (a boundary)
  return n * 2;                // return type inferred as number — no annotation needed
}

let a = 3;         // number (widened, because let is mutable)
const b = 3;       // 3 (literal type kept, because const can't change)

// ❌ any turns off the type checker silently
const raw: any = JSON.parse('{"n": 1}');
const wrong: number = raw.doesNotExist.atAll; // compiles! crashes at runtime.

// ✅ types are erased — this is what actually ships
function greet(name: string): string { return `hi ${name}`; }
// compiles to:  function greet(name) { return `hi ${name}`; }

// The boundary problem: TS trusts your annotation, the network doesn't
const user = await fetch('/api/me').then(r => r.json()); // r.json() is Promise<any>
// user is `any` — every property access is unchecked. Validate here (see Zod).