Object Types & Interfaces

Inline object types

You can annotate an object's shape directly, right where it's used.

let user: { name: string; age: number } = {
  name: "Ada",
  age: 36,
};
user.age = "old";
// Error: Type 'string' is not assignable to type 'number'.

Interfaces

An interface names a shape so you can reuse it across variables, parameters and return types.

interface User {
  name: string;
  age: number;
}

function printUser(u: User): void {
  console.log(`${u.name} (${u.age})`);
}

Why interfaces?

Any object with the right properties satisfies an interface — TS checks structure, not the class name. This is called "structural typing".