fix(MessageReaction): fetching a removed partial custom emoji

[izexi]

Fixes #3946

After a "partial" MessageReaction (being a custom emoji) was completely removed, the emoji getter would return an instance of GuildEmoji which was being passed into ReactionManager#_fetchReaction(). Since GuildEmoji#reaction will always return undefined it would end up throwing an error since _patch() was trying to be called on that

discord.js/src/managers/ReactionManager.js

Line 84 in 7994b56

reactionEmoji.reaction._patch({ count: 0 });

And as for why this doesn't occur within the messageReactionAdd event (which was asked on #library-discussion channel is because that line will only execute when a reaction emoji has been completely removed and/or when all reactions which is checked beforehand.

To fix this issue: instead of manually fetching the message and handling the fetched message to update the reactions Message#fetch() will be used to indirectly update the reaction, which is much simpler to how it was before so it can be done directly within MessageReaction#fetch() to avoid having trouble finding a reference externally to get access to the _patch method. It somewhat does the same by fetching the message and then updating Message#reactions (Message#fetch() → MessageManager#fetchId() → MessageManager#add() → Message#_patch() → ReactionManager#add() → ... MessageReaction#_patch()). When an emoji / all reactions has been completely removed it wouldn't be present within Message#reactions, so MessageReaction#count should obviously be set to 0
which is handled.

Here's a snippet of how I tested these changes

import { Client, DMChannel, ClientUser, Message } from 'discord.js';

interface MockEmoji {
  name: string;
  id: string | null;
}

const mockClient = new Client({ partials: ['MESSAGE', 'REACTION', 'CHANNEL'] });
const mockCustomEmoji: MockEmoji = { name: 'POGGERS', id: '4' };
const mockUnicodeEmoji: MockEmoji = { name: '😩', id: null };
const mockFetch = (reactions: { emoji: MockEmoji; count: number }[] = []) =>
  jest
    .spyOn(Message.prototype, 'fetch')
    .mockImplementation(async function(this: Message) {
      // @ts-ignore
      this._patch({
        reactions,
        id: this.id
      });
      return this;
    });

beforeAll(() => {
  // Add a mock ClientUser
  mockClient.user = new ClientUser(mockClient, { user: { id: '0' } });
  // Add a mock me
  mockClient.users.add({
    username: 'Havoc',
    id: '191615925336670208',
    discriminator: '7078',
    avatar: 'a_dde53f5afa943f4a2628d47045ef92c3'
  });
  // Add a mock channel
  mockClient.channels.add({
    id: '2',
    type: 1
  });
  // Add a partial message to the channel
  (mockClient.channels.cache.first() as DMChannel).messages.add({
    id: '3',
    channel_id: '2'
  });
});

describe('messageReactionAdd', () => {
  it('should fetch the custom reaction', done => {
    mockClient.once('messageReactionAdd', async reaction => {
      expect(reaction.partial).toBe(true);
      await reaction.fetch();
      expect(reaction.partial).toBe(false);
      expect(reaction.count).toBe(1);
      done();
    });
    mockFetch([{ emoji: mockCustomEmoji, count: 1 }]);
    // Mock a "partial" reaction being added to the message
    // @ts-ignore
    mockClient.actions.MessageReactionAdd.handle({
      user_id: '191615925336670208',
      emoji: mockCustomEmoji,
      message_id: '3',
      channel_id: '2'
    });
  });

  it('should fetch the unicode reaction', done => {
    mockClient.once('messageReactionAdd', async reaction => {
      expect(reaction.partial).toBe(true);
      await reaction.fetch();
      expect(reaction.partial).toBe(false);
      expect(reaction.count).toBe(1);
      done();
    });
    mockFetch([{ emoji: mockUnicodeEmoji, count: 1 }]);
    // Mock a "partial" reaction being added to the message
    // @ts-ignore
    mockClient.actions.MessageReactionAdd.handle({
      user_id: '191615925336670208',
      emoji: mockUnicodeEmoji,
      message_id: '3',
      channel_id: '2'
    });
  });
});

describe('messageReactionRemove', () => {
  it('should fetch the custom cleared reaction', done => {
    mockClient.once('messageReactionRemove', async reaction => {
      expect(reaction.partial).toBe(true);
      await reaction.fetch();
      expect(reaction.partial).toBe(false);
      expect(reaction.count).toBe(0);
      done();
    });
    mockFetch();
    // Mock a "partial" reaction being removed from the message
    // @ts-ignore
    mockClient.actions.MessageReactionRemove.handle({
      user_id: '191615925336670208',
      emoji: mockCustomEmoji,
      message_id: '3',
      channel_id: '2'
    });
  });

  it('should fetch the cleared reaction', done => {
    mockClient.once('messageReactionRemove', async reaction => {
      expect(reaction.partial).toBe(true);
      await reaction.fetch();
      expect(reaction.partial).toBe(false);
      expect(reaction.count).toBe(0);
      done();
    });
    mockFetch();
    // Mock a "partial" reaction being removed from the message
    // @ts-ignore
    mockClient.actions.MessageReactionRemove.handle({
      user_id: '191615925336670208',
      emoji: mockUnicodeEmoji,
      message_id: '3',
      channel_id: '2'
    });
  });
});

afterEach(() =>
  (mockClient.channels.cache.first() as DMChannel).messages.cache
    .first()
    ?.reactions.cache.clear()
);

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.