Readonly Properties & Index Signatures

Readonly Properties & Index Signatures

Interfaces support immutability rules and dynamic key definitions.

Readonly Properties

Mark properties with the readonly modifier to prevent reassignment after initialization.

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

const p: Point = { x: 10, y: 20 };
// p.x = 15; // Error: Cannot assign to 'x' because it is a read-only property.

Index Signatures

When object keys are unknown in advance, use Index Signatures:

interface StringDictionary {
  [key: string]: string;
}

const translations: StringDictionary = {
  hello: "Hola",
  goodbye: "Adiós",
  thankYou: "Gracias"
};

TL;DR

• readonly fields can only be set when the object is created.
• Index signatures allow defining objects with arbitrary key names matching a type pattern.