How to type objects in TypeScript with inline types

Typing objects inline

You can annotate an object's shape directly at the point of use:

let user: { name: string; age: number; isAdmin: boolean } = {
  name: "alice",
  age: 30,
  isAdmin: false
};

Optional properties

Add ? to mark a property as optional:

let config: { host: string; port?: number } = { host: "localhost" };
// port is undefined if not provided

Readonly properties

let point: { readonly x: number; readonly y: number } = { x: 0, y: 0 };
point.x = 5; // Error: Cannot assign to 'x' because it is a read-only property

Excess property checking

TypeScript rejects extra properties when you assign an object literal directly:

let user: { name: string } = { name: "alice", role: "admin" };
// Error: Object literal may only specify known properties

This only applies to direct assignments. Assigning through an intermediate variable bypasses the check — a subtle gotcha:

const tmp = { name: "alice", role: "admin" };
let user: { name: string } = tmp; // No error — structural check only

Inline object types are useful for one-off shapes. For reused shapes, use interfaces.