TypeScript - Structural Typing & Variance

Structural typing

type HasId = { id: string };

function takeId(x: HasId) {}

const obj = { id: 'x', extra: 1 };
takeId(obj); // ok (duck typing)

Assignment compatibility

type A = { a: number };
type B = { a: number; b: number };

const b: B = { a:1, b:2 };
const a: A = b; // ok (B is a superset)

Variance (high level)

• Function parameters are checked bivariantly by default (for compatibility).
• Use generic constraints and careful design to avoid unsafe substitutions.