Mapped Types

A mapped type is a type that transforms one type into another type by iterating over its properties.

Syntax

type NewType = {
    [Property in keyof OriginalType]: NewPropertyType 
}

• NewType: The new type that will be created
• Property: The property key
• OriginalType: The original type
• NewPropertyType: The new type of the property

Example: Make all properties readonly

interface Todo {
  title: string;
  completed: boolean;
}

type ReadonlyTodo = {
  readonly [Property in keyof Todo]: Todo[Property];
}

• Property in keyof Todo: This creates a new type by iterating over all properties of Todo
• Todo[Property]: This retrieves the type of each property Property from Todo

References

• Total TypeScript: Mapped Type
• Medium: Having Fun with TypeScript Mapped Types