TypeScript Tutorial · TypeScript Basics · Lesson 4 of 25

TypeScript Object Types and Type Aliases

Describe object shapes, optional and readonly properties, and name types with type.

Object types

Describe the properties an object must have:

Example 1Try it yourself

function describe(user: { name: string; age: number }): string {
  return `${user.name} is ${user.age}`;
}
console.log(describe({ name: "Ada", age: 36 }));

Output

Ada is 36

Optional and readonly properties

prop?: type — the property may be missing (its type includes undefined).
readonly prop: type — can't be reassigned after creation.

Type aliases

type Name = ... gives any type a reusable name:

Example 2Try it yourself

type Product = {
  readonly id: number;
  name: string;
  price: number;
  description?: string;
};

const mug: Product = { id: 1, name: "Mug", price: 9.5 };
mug.price = 8.5;
// @ts-expect-error — Cannot assign to 'id' because it is a read-only property.
mug.id = 2;  // types are compile-time only: if you ignore the error, the assignment still runs
console.log(mug, mug.description ?? "(no description)");

Output

{ id: 2, name: 'Mug', price: 8.5 } (no description)

Excess property checks

Object literals with unknown properties are rejected — this catches typos like emial:

Example 3Try it yourself

type Contact = { name: string; email: string };
// @ts-expect-error — Object literal may only specify known properties, and 'emial' does not exist.
const c: Contact = { name: "Ada", emial: "[email protected]" };
console.log(c);

Output

{ name: 'Ada', emial: '[email protected]' }

Test yourself

How do you mark an object property as optional?

name: string?
name?: string
optional name: string
name: string | optional

Show answer

Answer: name?: string