refactor(ThreadChannel): use single thread member endpoint

packages/discord.js/src/errors/Messages.js

@@ -95,7 +95,7 @@ const Messages = {
   [DjsErrorCodes.ChannelNotCached]: 'Could not find the channel where this message came from in the cache!',
   [DjsErrorCodes.StageChannelResolve]: 'Could not resolve channel to a stage channel.',
   [DjsErrorCodes.GuildScheduledEventResolve]: 'Could not resolve the guild scheduled event.',
-  [DjsErrorCodes.FetchOwnerId]: "Couldn't resolve the guild ownerId to fetch the member.",
+  [DjsErrorCodes.FetchOwnerId]: type => `Couldn't resolve the ${type} ownerId to fetch the ${type} member.`,
 
   [DjsErrorCodes.InvalidType]: (name, expected, an = false) => `Supplied ${name} is not a${an ? 'n' : ''} ${expected}.`,
   [DjsErrorCodes.InvalidElement]: (type, name, elem) => `Supplied ${type} ${name} includes an invalid element: ${elem}`,

packages/discord.js/src/structures/Guild.js

@@ -495,7 +495,7 @@ class Guild extends AnonymousGuild {
    */
   async fetchOwner(options) {
     if (!this.ownerId) {
-      throw new DiscordjsError(ErrorCodes.FetchOwnerId);
+      throw new DiscordjsError(ErrorCodes.FetchOwnerId, 'guild');
     }
     const member = await this.members.fetch({ ...options, user: this.ownerId });
     return member;

packages/discord.js/src/structures/ThreadChannel.js

@@ -1,11 +1,12 @@
 'use strict';
 
+const { DiscordAPIError } = require('@discordjs/rest');
 const { lazy } = require('@discordjs/util');
-const { ChannelType, PermissionFlagsBits, Routes, ChannelFlags } = require('discord-api-types/v10');
+const { RESTJSONErrorCodes, ChannelFlags, ChannelType, PermissionFlagsBits, Routes } = require('discord-api-types/v10');
 const { BaseChannel } = require('./BaseChannel');
 const getThreadOnlyChannel = lazy(() => require('./ThreadOnlyChannel'));
 const TextBasedChannel = require('./interfaces/TextBasedChannel');
-const { DiscordjsRangeError, ErrorCodes } = require('../errors');
+const { DiscordjsError, DiscordjsRangeError, ErrorCodes } = require('../errors');
 const GuildMessageManager = require('../managers/GuildMessageManager');
 const ThreadMemberManager = require('../managers/ThreadMemberManager');
 const ChannelFlagsBitField = require('../util/ChannelFlagsBitField');
@@ -293,15 +294,21 @@ class ThreadChannel extends BaseChannel {
    * @param {BaseFetchOptions} [options] The options for fetching the member
    * @returns {Promise<?ThreadMember>}
    */
-  async fetchOwner({ cache = true, force = false } = {}) {
-    if (!force) {
-      const existing = this.members.cache.get(this.ownerId);
-      if (existing) return existing;
+  async fetchOwner(options) {
+    if (!this.ownerId) {
+      throw new DiscordjsError(ErrorCodes.FetchOwnerId, 'thread');
     }
 
-    // We cannot fetch a single thread member, as of this commit's date, Discord API responds with 405
-    const members = await this.members.fetch({ cache });
-    return members.get(this.ownerId) ?? null;
+    // TODO: Remove that catch in the next major version
+    const member = await this.members._fetchSingle({ ...options, user: this.ownerId }).catch(error => {
+      if (error instanceof DiscordAPIError && error.code === RESTJSONErrorCodes.UnknownMember) {
+        return null;
+      }
+
+      throw error;
+    });
+
+    return member;
   }
 
   /**