Mapped Types

Mapped types in TypeScript are a way to create a new type based on an existing type, where each property of the existing type is transformed in some way. Mapped types are declared using a combination of the keyof operator and a type that maps each property of the existing type to a new property type.

For example, the following is a mapped type that takes an object type and creates a new type with all properties of the original type but with their type changed to readonly:

type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};

let obj = { x: 10, y: 20 };
let readonlyObj: Readonly<typeof obj> = obj;

In this example, the Readonly mapped type takes an object type T and creates a new type with all properties of T but with their type changed to readonly. The keyof T operator is used to extract the names of the properties of T, and the T[P] syntax is used to access the type of each property of T. The readonly keyword is used to make the properties of the new type readonly.

Learn more from the following links:

• Mapped Types