refactor: introduce trString for frontend (#38741)

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>

web_src/js/modules/i18n.ts

@@ -2,6 +2,21 @@ import {getCurrentLocale} from '../utils.ts';
 
 /** frontend `Locale.TrN`: pick the `_1` or `_n` form for `count` and interpolate `%d` */
 export function trN(count: number, form1: string, formN: string, {lang = getCurrentLocale()}: {lang?: string} = {}): string {
-  const form = new Intl.PluralRules(lang).select(count) === 'one' ? form1 : formN;
-  return form.replace('%d', String(count));
+  const text = new Intl.PluralRules(lang).select(count) === 'one' ? form1 : formN;
+  return trString(text, String(count));
+}
+
+/**
+ * Fills variables in a translated string (like TrString in Golang)
+ * Only `%s`, `%d` or one-based position like `%[1]s` are supported.
+ */
+export function trString(s: string, ...args: Array<string | number>) {
+  // the same behavior (almost) as backend TrString
+  let curIdx = 0;
+  return s.replace(/%%|%(?:\[([1-9]\d*)\])?([sd])/g, (match, indexed: string) => {
+    if (match === '%%') return '%';
+    const argIndex = indexed ? Number(indexed) - 1 : curIdx++;
+    if (argIndex < 0 || argIndex >= args.length) return match;
+    return String(args[argIndex]);
+  });
 }