Revert "remove unused file"

This reverts commit 9f78921.

String.format.js

@@ -0,0 +1,12 @@
+// add string format method
+if (!String.prototype.format) {
+  // eslint-disable-next-line no-extend-native
+  String.prototype.format = function () {
+    const args = arguments
+    return this.replace(/{(\d+)}/g, function (match, number) {
+      return typeof args[number] !== 'undefined'
+        ? args[number]
+        : match
+    })
+  }
+}