Add TypeScript Mapped Draft Type

[knpwrs]

These changes enable the following:

import produce from 'immer';
 
interface State {
  readonly x: number;
}
 
// `x` cannot be modified here
const state: State = {
  x: 0;
};
 
const newState = produce<State>((draft) => {
  // `x` can be modified here
  draft.x++;
});
 
// `newState.x` cannot be modified here

This helps TypeScript users ensure immutability of their state interfaces. I had to upgrade TypeScript for the index.d.ts file to parse properly. This feature was merged into TypeScript back in February: microsoft/TypeScript#21919

It also appears that the flow/ts.ts file doesn't run and never did. I tested this externally and it works, but I'd like to see a more robust test suite in this repo.

EDIT: I've added some TypeScript tests in Jest.

EDIT: Fixes #97

[coveralls]

Coverage remained the same at 93.137% when pulling 1978d62 on knpwrs:mapped-draft-type into 537aa92 on mweststrate:master.

[knpwrs]

I just pushed a commit to make the Draft type recursive so users can have nested readonly interfaces:

interface State {
  readonly foo: number;
  readonly baz: {
    readonly x: number;
    readonly y: number;
  };
}

I'm still trying to figure out how to make arrays work properly.

[knpwrs]

Looks like everything is working now. All of the following compiles and works as expected:

import produce from 'immer';

interface State {
  readonly num: number;
  readonly foo?: string;
  bar: string;
  readonly baz: {
    readonly x: number;
    readonly y: number;
  };
  readonly arr: ReadonlyArray<{ readonly value: string }>;
  readonly arr2: { readonly value: string }[];
}

const state: State = {
  num: 0,
  bar: 'foo',
  baz: {
    x: 1,
    y: 2,
  },
  arr: [{ value: 'asdf' }],
  arr2: [{ value: 'asdf' }],
}

const newState = produce<State>(draft => {
  draft.num++;
  draft.foo = 'bar';
  draft.bar = 'foo';
  draft.baz.x++;
  draft.baz.y++;
  draft.arr[0].value = 'foo';
  draft.arr.push({ value: 'asf' });
  draft.arr2[0].value = 'foo';
  draft.arr2.push({ value: 'asf' });
})(state);

[knpwrs]

I only just now noticed the previous discussions in #97 and #109. I believe what I have here is a solution that fixes #97 and addresses the problems in #109.

[ghost]

This would be really useful to us, thank you very much @knpwrs

[knpwrs]

@mweststrate is there any interest from the project owners to add these typings?

[mweststrate]

Yes; but didn't get to reviewing yet 😊 Op za 23 jun. 2018 19:42 schreef Ken Powers <notifications@github.com>:

…

@mweststrate <https://github.com/mweststrate> is there any interest from the project owners to add these typings? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#161 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABvGhFRsW1xcoEuuMj_GEeYBBdiVUNHAks5t_n4igaJpZM4UhG-U> .

[ghost]

Hello @mweststrate
+1 for this PR from my side 😀 👍

[mweststrate]

Reviewed! No comments, looks awesome :). Will release next week