What Is TypeScript & Why It Exists

// ── JavaScript (no types - errors only at runtime) ────────────
function greet(name) {
  return "Hello, " + name.toUpperCase();
}
greet(42); // 💥 Runtime crash: name.toUpperCase is not a function

// ── TypeScript (types enforced at compile time) ────────────────
function greet(name: string): string {
  return "Hello, " + name.toUpperCase();
}
greet(42);
// ❌ Compile error: Argument of type 'number' is not assignable
//    to parameter of type 'string'.
// → Caught BEFORE the code ever runs.

// ── Type Erasure - TypeScript disappears at runtime ────────────
// TypeScript source:
interface User {
  id: number;
  name: string;
}
const user: User = { id: 1, name: "Alice" };

// Compiled JavaScript output - the interface is GONE:
// const user = { id: 1, name: "Alice" };
//
// The runtime (browser / Node.js) has NO knowledge of `User`.
// This is why runtime validation (typeof, Zod) is necessary
// at boundaries like API responses and form inputs.