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/games/reversi/core.ts b/src/games/reversi/core.ts
index b610d46884..34eb03becb 100644
--- a/src/games/reversi/core.ts
+++ b/src/games/reversi/core.ts
@@ -1,3 +1,5 @@
+import { count } from "../../prelude/array";
+
 // MISSKEY REVERSI ENGINE
 
 /**
@@ -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);
 	}
 
 	/**
diff --git a/src/prelude/array.ts b/src/prelude/array.ts
new file mode 100644
index 0000000000..e944030a7f
--- /dev/null
+++ b/src/prelude/array.ts
@@ -0,0 +1,7 @@
+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);
+}
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 2458a70556..39c385853d 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: {
@@ -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/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';
 	}