What is the difference between any, unknown, and never in TypeScript?

These three types occupy opposite ends of the type hierarchy.

any — the escape hatch. Disables all type checking for that value. Assignable to and from everything.

let a: any = 42;
a.foo.bar.baz; // no error — type checking disabled
const b: number = a; // OK — any is assignable to anything

unknown — the type-safe alternative to any. You must narrow it before using it.

let u: unknown = getValueFromAPI();

u.toUpperCase(); // ❌ Error — must narrow first

if (typeof u === 'string') {
  u.toUpperCase(); // ✅ Safe — narrowed to string
}

// unknown is NOT assignable to specific types without narrowing
const s: string = u; // ❌ Error

never — the bottom type. A value that can never exist. Used for:

• Functions that never return (throw or infinite loop)
• Exhaustive checks in switch/if statements
• Impossible intersections

function fail(msg: string): never {
  throw new Error(msg); // never returns
}

// Exhaustive check — never signals unhandled case
type Shape = 'circle' | 'square';
function area(s: Shape) {
  switch (s) {
    case 'circle': return Math.PI;
    case 'square': return 1;
    default:
      const _exhaustive: never = s; // ❌ Error if new case not handled
  }
}