TypeScript - Removing readonly modifier

There's a way:

type Writeable<T extends { [x: string]: any }, K extends string> = {
    [P in K]: T[P];
}

(code in playground)

But you can go the opposite way and it will make things much easier:

interface Foo {
    bar: boolean;
}

type ReadonlyFoo = Readonly<Foo>;

let baz: Foo;
baz.bar = true; // fine

(baz as ReadonlyFoo).bar = true; // error

(code in playground)

Update

Since typescript 2.8 there's a new way to do it:

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

If you need your type to be writeable recursively, then:

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

These type definitions are called mapped types