Optional & Readonly Properties

Optional properties

A ? after the property name means it may be missing from the object entirely.

interface Config {
  host: string;
  port?: number;   // optional
}

const c1: Config = { host: "localhost" };        // OK
const c2: Config = { host: "localhost", port: 8080 }; // OK

readonly properties

readonly allows setting a property once (usually at creation) but blocks any later reassignment.

interface Point {
  readonly x: number;
  readonly y: number;
}

const p: Point = { x: 0, y: 0 };
p.x = 10;
// Error: Cannot assign to 'x' because it is a read-only property.

readonly is compile-time only

readonly is erased at compile time — it protects you in TS code, not against JS code that ignores types at runtime.