diff --git a/src/client/app/common/scripts/gcd.ts b/src/client/app/common/scripts/gcd.ts
deleted file mode 100644
index 9a19f9da66..0000000000
--- a/src/client/app/common/scripts/gcd.ts
+++ /dev/null
@@ -1,2 +0,0 @@
-const gcd = (a, b) => !b ? a : gcd(b, a % b);
-export default gcd;
diff --git a/src/client/app/common/views/components/acct.vue b/src/client/app/common/views/components/acct.vue
index 1ad222afdd..2b5efe609c 100644
--- a/src/client/app/common/views/components/acct.vue
+++ b/src/client/app/common/views/components/acct.vue
@@ -1,14 +1,20 @@
 <template>
 <span class="mk-acct">
 	<span class="name">@{{ user.username }}</span>
-	<span class="host" v-if="user.host">@{{ user.host }}</span>
+	<span class="host" v-if="user.host || detail">@{{ user.host || host }}</span>
 </span>
 </template>
 
 <script lang="ts">
 import Vue from 'vue';
+import { host } from '../../../config';
 export default Vue.extend({
-	props: ['user']
+	props: ['user', 'detail'],
+	data() {
+		return {
+			host
+		};
+	}
 });
 </script>
 
diff --git a/src/client/app/desktop/views/pages/deck/deck.column.vue b/src/client/app/desktop/views/pages/deck/deck.column.vue
index 239b1b0447..abb09775fb 100644
--- a/src/client/app/desktop/views/pages/deck/deck.column.vue
+++ b/src/client/app/desktop/views/pages/deck/deck.column.vue
@@ -28,6 +28,7 @@
 import Vue from 'vue';
 import Menu from '../../../../common/views/components/menu.vue';
 import contextmenu from '../../../api/contextmenu';
+import { countIf } from '../../../../../../prelude/array';
 
 export default Vue.extend({
 	props: {
@@ -117,7 +118,7 @@ export default Vue.extend({
 		toggleActive() {
 			if (!this.isStacked) return;
 			const vms = this.$store.state.settings.deck.layout.find(ids => ids.indexOf(this.column.id) != -1).map(id => this.getColumnVm(id));
-			if (this.active && vms.filter(vm => vm.$el.classList.contains('active')).length == 1) return;
+			if (this.active && countIf(vm => vm.$el.classList.contains('active'), vms) == 1) return;
 			this.active = !this.active;
 		},
 
diff --git a/src/client/app/desktop/views/pages/user/user.header.vue b/src/client/app/desktop/views/pages/user/user.header.vue
index d8f4656ed0..4b434ec219 100644
--- a/src/client/app/desktop/views/pages/user/user.header.vue
+++ b/src/client/app/desktop/views/pages/user/user.header.vue
@@ -6,7 +6,7 @@
 		<div class="title">
 			<p class="name">{{ user | userName }}</p>
 			<div>
-				<span class="username"><mk-acct :user="user"/></span>
+				<span class="username"><mk-acct :user="user" :detail="true" /></span>
 				<span v-if="user.isBot" title="%i18n:@is-bot%">%fa:robot%</span>
 				<span class="location" v-if="user.host === null && user.profile.location">%fa:map-marker% {{ user.profile.location }}</span>
 				<span class="birthday" v-if="user.host === null && user.profile.birthday">%fa:birthday-cake% {{ user.profile.birthday.replace('-', '年').replace('-', '月') + '日' }} ({{ age }}歳)</span>
diff --git a/src/client/app/mobile/views/components/drive.file-detail.vue b/src/client/app/mobile/views/components/drive.file-detail.vue
index 43867211e9..8108892597 100644
--- a/src/client/app/mobile/views/components/drive.file-detail.vue
+++ b/src/client/app/mobile/views/components/drive.file-detail.vue
@@ -67,7 +67,7 @@
 import Vue from 'vue';
 import * as EXIF from 'exif-js';
 import * as hljs from 'highlight.js';
-import gcd from '../../../common/scripts/gcd';
+import { gcd } from '../../../../../prelude/math';
 
 export default Vue.extend({
 	props: ['file'],
diff --git a/src/client/app/mobile/views/pages/user.vue b/src/client/app/mobile/views/pages/user.vue
index ddea43c9f2..c1082f31a9 100644
--- a/src/client/app/mobile/views/pages/user.vue
+++ b/src/client/app/mobile/views/pages/user.vue
@@ -16,7 +16,7 @@
 				</div>
 				<div class="title">
 					<h1>{{ user | userName }}</h1>
-					<span class="username"><mk-acct :user="user"/></span>
+					<span class="username"><mk-acct :user="user" :detail="true" /></span>
 					<span class="followed" v-if="user.isFollowed">%i18n:@follows-you%</span>
 				</div>
 				<div class="description">
diff --git a/src/games/reversi/core.ts b/src/games/reversi/core.ts
index b610d46884..9199efa092 100644
--- a/src/games/reversi/core.ts
+++ b/src/games/reversi/core.ts
@@ -1,3 +1,5 @@
+import { count, countIf } from "../../prelude/array";
+
 // MISSKEY REVERSI ENGINE
 
 /**
@@ -88,8 +90,8 @@ export default class Reversi {
 		//#endregion
 
 		// ゲームが始まった時点で片方の色の石しかないか、始まった時点で勝敗が決定するようなマップの場合がある
-		if (this.canPutSomewhere(BLACK).length == 0) {
-			if (this.canPutSomewhere(WHITE).length == 0) {
+		if (!this.canPutSomewhere(BLACK)) {
+			if (!this.canPutSomewhere(WHITE)) {
 				this.turn = null;
 			} else {
 				this.turn = WHITE;
@@ -101,14 +103,14 @@ export default class Reversi {
 	 * 黒石の数
 	 */
 	public get blackCount() {
-		return this.board.filter(x => x === BLACK).length;
+		return count(BLACK, this.board);
 	}
 
 	/**
 	 * 白石の数
 	 */
 	public get whiteCount() {
-		return this.board.filter(x => x === WHITE).length;
+		return count(BLACK, this.board);
 	}
 
 	/**
@@ -170,9 +172,9 @@ export default class Reversi {
 
 	private calcTurn() {
 		// ターン計算
-		if (this.canPutSomewhere(!this.prevColor).length > 0) {
+		if (this.canPutSomewhere(!this.prevColor)) {
 			this.turn = !this.prevColor;
-		} else if (this.canPutSomewhere(this.prevColor).length > 0) {
+		} else if (this.canPutSomewhere(this.prevColor)) {
 			this.turn = this.prevColor;
 		} else {
 			this.turn = null;
@@ -204,10 +206,17 @@ export default class Reversi {
 	/**
 	 * 打つことができる場所を取得します
 	 */
-	public canPutSomewhere(color: Color): number[] {
+	public puttablePlaces(color: Color): number[] {
 		return Array.from(this.board.keys()).filter(i => this.canPut(color, i));
 	}
 
+	/**
+	 * 打つことができる場所があるかどうかを取得します
+	 */
+	public canPutSomewhere(color: Color): boolean {
+		return this.puttablePlaces(color).length > 0;
+	}
+
 	/**
 	 * 指定のマスに石を打つことができるかどうかを取得します
 	 * @param color 自分の色
diff --git a/src/mfm/html.ts b/src/mfm/html.ts
index 2e38fe10a0..b7fa5b6f03 100644
--- a/src/mfm/html.ts
+++ b/src/mfm/html.ts
@@ -4,10 +4,7 @@ const { JSDOM } = jsdom;
 import config from '../config';
 import { INote } from '../models/note';
 import { TextElement } from './parse';
-
-function intersperse<T>(sep: T, xs: T[]): T[] {
-	return [].concat(...xs.map(x => [sep, x])).slice(1);
-}
+import { intersperse } from '../prelude/array';
 
 const handlers: { [key: string]: (window: any, token: any, mentionedRemoteUsers: INote['mentionedRemoteUsers']) => void } = {
 	bold({ document }, { bold }) {
diff --git a/src/prelude/README.md b/src/prelude/README.md
new file mode 100644
index 0000000000..bb728cfb1b
--- /dev/null
+++ b/src/prelude/README.md
@@ -0,0 +1,3 @@
+# Prelude
+このディレクトリのコードはJavaScriptの表現能力を補うためのコードです。
+Misskey固有の処理とは独立したコードの集まりですが、Misskeyのコードを読みやすくすることを目的としています。
diff --git a/src/prelude/array.ts b/src/prelude/array.ts
new file mode 100644
index 0000000000..aee17640ed
--- /dev/null
+++ b/src/prelude/array.ts
@@ -0,0 +1,11 @@
+export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
+	return xs.filter(f).length;
+}
+
+export function count<T>(x: T, xs: T[]): number {
+	return countIf(y => x === y, xs);
+}
+
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return [].concat(...xs.map(x => [sep, x])).slice(1);
+}
diff --git a/src/prelude/math.ts b/src/prelude/math.ts
new file mode 100644
index 0000000000..07b94bec30
--- /dev/null
+++ b/src/prelude/math.ts
@@ -0,0 +1,3 @@
+export function gcd(a: number, b: number): number {
+	return b === 0 ? a : gcd(b, a % b);
+}
diff --git a/src/server/activitypub/outbox.ts b/src/server/activitypub/outbox.ts
index cc7e55b5df..a5e762eea8 100644
--- a/src/server/activitypub/outbox.ts
+++ b/src/server/activitypub/outbox.ts
@@ -10,6 +10,7 @@ import { setResponseType } from '../activitypub';
 
 import Note from '../../models/note';
 import renderNote from '../../remote/activitypub/renderer/note';
+import { countIf } from '../../prelude/array';
 
 export default async (ctx: Router.IRouterContext) => {
 	const userId = new mongo.ObjectID(ctx.params.user);
@@ -25,7 +26,7 @@ export default async (ctx: Router.IRouterContext) => {
 	const page: boolean = ctx.request.query.page === 'true';
 
 	// Validate parameters
-	if (sinceIdErr || untilIdErr || pageErr || [sinceId, untilId].filter(x => x != null).length > 1) {
+	if (sinceIdErr || untilIdErr || pageErr || countIf(x => x != null, [sinceId, untilId]) > 1) {
 		ctx.status = 400;
 		return;
 	}
diff --git a/src/server/api/endpoints/notes/global-timeline.ts b/src/server/api/endpoints/notes/global-timeline.ts
index e70fc5d76f..5d93cd78ec 100644
--- a/src/server/api/endpoints/notes/global-timeline.ts
+++ b/src/server/api/endpoints/notes/global-timeline.ts
@@ -4,6 +4,7 @@ import Mute from '../../../../models/mute';
 import { pack } from '../../../../models/note';
 import { ILocalUser } from '../../../../models/user';
 import getParams from '../../get-params';
+import { countIf } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -42,7 +43,7 @@ export default async (params: any, user: ILocalUser) => {
 	if (psErr) throw psErr;
 
 	// Check if only one of sinceId, untilId, sinceDate, untilDate specified
-	if ([ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate].filter(x => x != null).length > 1) {
+	if (countIf(x => x != null, [ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate]) > 1) {
 		throw 'only one of sinceId, untilId, sinceDate, untilDate can be specified';
 	}
 
diff --git a/src/server/api/endpoints/notes/hybrid-timeline.ts b/src/server/api/endpoints/notes/hybrid-timeline.ts
index 16cec86797..0eb7b61830 100644
--- a/src/server/api/endpoints/notes/hybrid-timeline.ts
+++ b/src/server/api/endpoints/notes/hybrid-timeline.ts
@@ -5,6 +5,7 @@ import { getFriends } from '../../common/get-friends';
 import { pack } from '../../../../models/note';
 import { ILocalUser } from '../../../../models/user';
 import getParams from '../../get-params';
+import { countIf } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -86,7 +87,7 @@ export default async (params: any, user: ILocalUser) => {
 	if (psErr) throw psErr;
 
 	// Check if only one of sinceId, untilId, sinceDate, untilDate specified
-	if ([ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate].filter(x => x != null).length > 1) {
+	if (countIf(x => x != null, [ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate]) > 1) {
 		throw 'only one of sinceId, untilId, sinceDate, untilDate can be specified';
 	}
 
diff --git a/src/server/api/endpoints/notes/local-timeline.ts b/src/server/api/endpoints/notes/local-timeline.ts
index ce84b4135a..ff10e6fbaa 100644
--- a/src/server/api/endpoints/notes/local-timeline.ts
+++ b/src/server/api/endpoints/notes/local-timeline.ts
@@ -4,6 +4,7 @@ import Mute from '../../../../models/mute';
 import { pack } from '../../../../models/note';
 import { ILocalUser } from '../../../../models/user';
 import getParams from '../../get-params';
+import { countIf } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -48,7 +49,7 @@ export default async (params: any, user: ILocalUser) => {
 	if (psErr) throw psErr;
 
 	// Check if only one of sinceId, untilId, sinceDate, untilDate specified
-	if ([ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate].filter(x => x != null).length > 1) {
+	if (countIf(x => x != null, [ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate]) > 1) {
 		throw 'only one of sinceId, untilId, sinceDate, untilDate can be specified';
 	}
 
diff --git a/src/server/api/endpoints/notes/timeline.ts b/src/server/api/endpoints/notes/timeline.ts
index 089e7a182a..5f3844987c 100644
--- a/src/server/api/endpoints/notes/timeline.ts
+++ b/src/server/api/endpoints/notes/timeline.ts
@@ -5,6 +5,7 @@ import { getFriends } from '../../common/get-friends';
 import { pack } from '../../../../models/note';
 import { ILocalUser } from '../../../../models/user';
 import getParams from '../../get-params';
+import { countIf } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -86,7 +87,7 @@ export default async (params: any, user: ILocalUser) => {
 	if (psErr) throw psErr;
 
 	// Check if only one of sinceId, untilId, sinceDate, untilDate specified
-	if ([ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate].filter(x => x != null).length > 1) {
+	if (countIf(x => x != null, [ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate]) > 1) {
 		throw 'only one of sinceId, untilId, sinceDate, untilDate can be specified';
 	}
 
diff --git a/src/server/api/endpoints/users/notes.ts b/src/server/api/endpoints/users/notes.ts
index 42c31189d6..1ab7786a18 100644
--- a/src/server/api/endpoints/users/notes.ts
+++ b/src/server/api/endpoints/users/notes.ts
@@ -3,6 +3,7 @@ import getHostLower from '../../common/get-host-lower';
 import Note, { pack } from '../../../../models/note';
 import User, { ILocalUser } from '../../../../models/user';
 import getParams from '../../get-params';
+import { countIf } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -110,7 +111,7 @@ export default (params: any, me: ILocalUser) => new Promise(async (res, rej) =>
 	}
 
 	// Check if only one of sinceId, untilId, sinceDate, untilDate specified
-	if ([ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate].filter(x => x != null).length > 1) {
+	if (countIf(x => x != null, [ps.sinceId, ps.untilId, ps.sinceDate, ps.untilDate]) > 1) {
 		throw 'only one of sinceId, untilId, sinceDate, untilDate can be specified';
 	}