[TypeScript] Modifier

TypeScript 2.8 adds the ability for a mapped type to either add or remove a particular modifier. Specifically, a readonly or ? property modifier in a mapped type can now be prefixed with either + or - to indicate that the modifier should be added or removed.

type MutableRequired<T> = { -readonly [P in keyof T]-?: T[P] };  // Remove readonly and ?

type ReadonlyPartial<T> = { +readonly [P in keyof T]+?: T[P] };  // Add readonly and ?

Example:

type MutableRequired<T> = T extends object ? {-readonly [K in keyof T]: T[K]} : T;

interface Book {
    readonly name: String;
}

const newState: MutableRequired<Book> = {
    name: 'st'
};

newState's anme is mutable and required.