diff --git a/packages/fineui/src/polyfill/event.js b/packages/fineui/src/polyfill/event.js
index 346ec16c8..decca0f7f 100644
--- a/packages/fineui/src/polyfill/event.js
+++ b/packages/fineui/src/polyfill/event.js
@@ -1,9 +1,9 @@
-(function () {
-    if (typeof window.CustomEvent === "function") return false; // If not IE
+const isIE = () => typeof window.CustomEvent !== "function";
 
-    function CustomEvent (event, params) {
+if (isIE()) {
+    function CustomEvent(event, params) {
         params = params || { bubbles: false, cancelable: false, detail: undefined };
-        var evt = document.createEvent("CustomEvent");
+        const evt = document.createEvent("CustomEvent");
         evt.initCustomEvent(event, params.bubbles, params.cancelable, params.detail);
 
         return evt;
@@ -12,4 +12,4 @@
     CustomEvent.prototype = window.Event.prototype;
 
     window.CustomEvent = CustomEvent;
-}());
+}