refactoring

Resolve #7779

packages/backend/src/prelude/string.ts

@@ -0,0 +1,15 @@
+export function concat(xs: string[]): string {
+	return xs.join('');
+}
+
+export function capitalize(s: string): string {
+	return toUpperCase(s.charAt(0)) + toLowerCase(s.slice(1));
+}
+
+export function toUpperCase(s: string): string {
+	return s.toUpperCase();
+}
+
+export function toLowerCase(s: string): string {
+	return s.toLowerCase();
+}