types(Partials): add toString() method to supported Partials (#9835)

* types(Partials): add toString() method to supported Partials

- includes `PartialDMChannel`, `PartialGuildMember` and `PartialUser`
- does not include
    - `PartialMessage`, since `<PartialMessage>.content` is always null
    - `PartialMessageReaction`, since `MessageReaction` has no `toString()` method
    - `PartialThreadMember`, since `ThreadMember` has no `toString()` method

* types(Partials): replace type of `toString()` methods with return type

* test(Partials): add tests

* types(Partialize): refactor Partialize

* test(PartialThreadMember): fix typo

* types(Partials): clean up unnecessary type overrides

* test(Partials): add tests for <Partials>.partial property

---------

Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>

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

@@ -6161,15 +6161,17 @@ export interface PartialChannelData {
 
 export type Partialize<
   T extends AllowedPartial,
-  N extends keyof T | null = null,
+  NulledKeys extends keyof T | null = null,
-  M extends keyof T | null = null,
+  NullableKeys extends keyof T | null = null,
-  E extends keyof T | '' = '',
+  OverridableKeys extends keyof T | '' = '',
 > = {
-  readonly client: Client<true>;
+  [K in keyof Omit<T, OverridableKeys>]: K extends 'partial'
-  id: Snowflake;
+    ? true
-  partial: true;
+    : K extends NulledKeys
-} & {
+    ? null
-  [K in keyof Omit<T, 'client' | 'id' | 'partial' | E>]: K extends N ? null : K extends M ? T[K] | null : T[K];
+    : K extends NullableKeys
+    ? T[K] | null
+    : T[K];
 };
 
 export interface PartialDMChannel extends Partialize<DMChannel, null, null, 'lastMessageId'> {

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

@@ -177,6 +177,10 @@ import {
   StringSelectMenuComponentData,
   ButtonComponentData,
   MediaChannel,
+  PartialDMChannel,
+  PartialGuildMember,
+  PartialMessage,
+  PartialMessageReaction,
 } from '.';
 import { expectAssignable, expectNotAssignable, expectNotType, expectType } from 'tsd';
 import type { ContextMenuCommandBuilder, SlashCommandBuilder } from '@discordjs/builders';
@@ -2323,3 +2327,39 @@ expectType<Readonly<GuildMemberFlagsBitField>>(guildMember.flags);
   const onboarding = await guild.fetchOnboarding();
   expectType<GuildOnboarding>(onboarding);
 }
+
+declare const partialDMChannel: PartialDMChannel;
+expectType<true>(partialDMChannel.partial);
+expectType<undefined>(partialDMChannel.lastMessageId);
+
+declare const partialGuildMember: PartialGuildMember;
+expectType<true>(partialGuildMember.partial);
+expectType<null>(partialGuildMember.joinedAt);
+expectType<null>(partialGuildMember.joinedTimestamp);
+expectType<null>(partialGuildMember.pending);
+
+declare const partialMessage: PartialMessage;
+expectType<true>(partialMessage.partial);
+expectType<null>(partialMessage.type);
+expectType<null>(partialMessage.system);
+expectType<null>(partialMessage.pinned);
+expectType<null>(partialMessage.tts);
+expectAssignable<null | Message['content']>(partialMessage.content);
+expectAssignable<null | Message['cleanContent']>(partialMessage.cleanContent);
+expectAssignable<null | Message['author']>(partialMessage.author);
+
+declare const partialMessageReaction: PartialMessageReaction;
+expectType<true>(partialMessageReaction.partial);
+expectType<null>(partialMessageReaction.count);
+
+declare const partialThreadMember: PartialThreadMember;
+expectType<true>(partialThreadMember.partial);
+expectType<null>(partialThreadMember.flags);
+expectType<null>(partialThreadMember.joinedAt);
+expectType<null>(partialThreadMember.joinedTimestamp);
+
+declare const partialUser: PartialUser;
+expectType<true>(partialUser.partial);
+expectType<null>(partialUser.username);
+expectType<null>(partialUser.tag);
+expectType<null>(partialUser.discriminator);