diff --git a/lib/plugins/helper/fragment_cache.js b/lib/plugins/helper/fragment_cache.js
index b54468b372..6510b8de7b 100644
--- a/lib/plugins/helper/fragment_cache.js
+++ b/lib/plugins/helper/fragment_cache.js
@@ -3,10 +3,8 @@
 module.exports = ctx => {
   let cache = {};
 
-  ctx.on('generateBefore', function() {
-    // reset cache for watch mode
-    cache = {};
-  });
+  // reset cache for watch mode
+  ctx.on('generateBefore', () => { cache = {}; });
 
   return function fragmentCache(id, fn) {
     if (this.cache && cache[id] != null) return cache[id];