What are mapped types? Show how `Partial` or `Readonly` could be implemented.

Summary

Mapped types create new types by iterating over keys: { [K in keyof T]: ... }. Modifiers ?, readonly, -? and -readonly add or remove optionality and readonly-ness, and as clauses remap or filter keys.

Longer Answer

Key remapping with template literal types enables things like generating getter names. Filtering keys uses as with a conditional resolving to never.

Code Example

type MyPartial<T> = { [K in keyof T]?: T[K] };
type Mutable<T> = { -readonly [K in keyof T]: T[K] };

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};
type UserGetters = Getters<{ name: string; age: number }>;
// { getName: () => string; getAge: () => number }

type OnlyStrings<T> = { [K in keyof T as T[K] extends string ? K : never]: T[K] };

Key Points

• [K in keyof T] iteration
• ?/readonly modifiers and - removal
• as for key remapping/filtering