diff --git a/src/client/app/desktop/views/pages/welcome.vue b/src/client/app/desktop/views/pages/welcome.vue
index 0138fde273..7409f6c9b6 100644
--- a/src/client/app/desktop/views/pages/welcome.vue
+++ b/src/client/app/desktop/views/pages/welcome.vue
@@ -85,6 +85,7 @@
 <script lang="ts">
 import Vue from 'vue';
 import { host, copyright } from '../../../config';
+import { concat } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	data() {
@@ -119,8 +120,8 @@ export default Vue.extend({
 		(this as any).api('notes/local-timeline', {
 			fileType: image,
 			limit: 6
-		}).then(notes => {
-			const files = [].concat(...notes.map(n => n.files));
+		}).then((notes: any[]) => {
+			const files = concat(notes.map((n: any): any[] => n.files));
 			this.photos = files.filter(f => image.includes(f.type)).slice(0, 6);
 		});
 	},
diff --git a/src/client/app/mobile/views/pages/welcome.vue b/src/client/app/mobile/views/pages/welcome.vue
index 1856731d8a..7446cc700f 100644
--- a/src/client/app/mobile/views/pages/welcome.vue
+++ b/src/client/app/mobile/views/pages/welcome.vue
@@ -40,6 +40,7 @@
 <script lang="ts">
 import Vue from 'vue';
 import { apiUrl, copyright, host } from '../../../config';
+import { concat } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	data() {
@@ -79,8 +80,8 @@ export default Vue.extend({
 		(this as any).api('notes/local-timeline', {
 			fileType: image,
 			limit: 6
-		}).then(notes => {
-			const files = [].concat(...notes.map(n => n.files));
+		}).then((notes: any[]) => {
+			const files = concat(notes.map((n: any): any[] => n.files));
 			this.photos = files.filter(f => image.includes(f.type)).slice(0, 6);
 		});
 	}
diff --git a/src/games/reversi/core.ts b/src/games/reversi/core.ts
index e2a2289f18..3ca9d052af 100644
--- a/src/games/reversi/core.ts
+++ b/src/games/reversi/core.ts
@@ -1,4 +1,4 @@
-import { count, countIf } from "../../prelude/array";
+import { count, concat } from "../../prelude/array";
 
 // MISSKEY REVERSI ENGINE
 
@@ -286,7 +286,7 @@ export default class Reversi {
 			}
 		};
 
-		return [].concat(...diffVectors.map(effectsInLine));
+		return concat(diffVectors.map(effectsInLine));
 	}
 
 	/**
diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index aee17640ed..9a3c266d6d 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -6,6 +6,10 @@ 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);
+export function concat<T>(xss: T[][]): T[] {
+	return ([] as T[]).concat(...xss);
+}
+
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return concat(xs.map(x => [sep, x])).slice(1);
 }