[TypeScript] Produced draft is read-only if original state is read-only

[olavim]

The draft returned by the produce function's callback should make a mutable clone from the original state. Currently immutability is retained:

interface IState {
  readonly foo: boolean;
}

const state: IState = {
  foo: false
};

produce(state, draft => {
  draft.foo = true; // Error: Cannot assign to 'foo' because it is a constant or a read-only property.
});

[olavim]

I submitted a PR to fix this.

As a quick solution until this is fixed in a release:

// custom.d.ts

declare module 'immer' {
  export type MutableState<T extends {[x: string]: any}, K extends string> = {
    [P in K]: T[P];
  };

  export type Recipe<S> = (this: S, draftState: MutableState<S, keyof S>) => void;

  export default function<S = any>(
    currentState: S,
    recipe: Recipe<S>
  ): S;
}

This is fixed by #109

[mweststrate]

Released as 1.4.0. Have fun!