fix: readd isThread type guard (#8019)

packages/discord.js/src/structures/Channel.js

@@ -3,6 +3,7 @@
 const { DiscordSnowflake } = require('@sapphire/snowflake');
 const { ChannelType, Routes } = require('discord-api-types/v10');
 const Base = require('./Base');
+const { ThreadChannelTypes } = require('../util/Constants');
 let CategoryChannel;
 let DMChannel;
 let NewsChannel;
@@ -109,6 +110,14 @@ class Channel extends Base {
     return this.client.channels.fetch(this.id, { force });
   }
 
+  /**
+   * Indicates whether this channel is a {@link ThreadChannel}.
+   * @returns {boolean}
+   */
+  isThread() {
+    return ThreadChannelTypes.includes(this.type);
+  }
+
   /**
    * Indicates whether this channel is {@link TextBasedChannels text-based}.
    * @returns {boolean}

packages/discord.js/typings/index.d.ts

@@ -750,6 +750,7 @@ export abstract class Channel extends Base {
   public get url(): string;
   public delete(): Promise<this>;
   public fetch(force?: boolean): Promise<this>;
+  public isThread(): this is ThreadChannel;
   public isTextBased(): this is TextBasedChannel;
   public isDMBased(): this is PartialGroupDMChannel | DMChannel | PartialDMChannel;
   public isVoiceBased(): this is VoiceBasedChannel;

packages/discord.js/typings/index.test-d.ts

@@ -557,7 +557,7 @@ client.on('guildCreate', async g => {
     channel.send({ components: [row, row2] });
   }
 
-  if (channel.type === ChannelType.GuildPublicThread) {
+  if (channel.isThread()) {
     const fetchedMember = await channel.members.fetch({ member: '12345678' });
     expectType<ThreadMember>(fetchedMember);
     const fetchedMemberCol = await channel.members.fetch(true);