Structural typing

Yesterday, in one sentence

Interview question

TypeScript decides whether two types are compatible by comparing their shape, not their name. This is called structural typing, and it is sometimes called duck typing: if it looks like a duck, it is a duck.

interface Mutant {
  id: number;
  power: string;
}

type Magic = {
  id: number;
  power: string;
};

const juggernaut: Magic = { id: 1, power: "unstoppable" };

const mutant: Mutant = juggernaut; // ok, the shapes match

Mutant was declared as an interfaceInterface and Magic as a type aliasType Alias, but they have the same properties with the same types, so a value of one is assignable to the other.

Check yourself

interface Peter {
  name: string
  city: string
}

type Miles = {
  name: string
  city: string
}