feat(fragment_cache): bring up lru

lib/plugins/helper/fragment_cache.js

@@ -1,16 +1,18 @@
 'use strict';
 
+const LRU = require('lru-cache');
+
 module.exports = ctx => {
-  let cache = {};
+  const cache = new LRU(50);
 
   // reset cache for watch mode
-  ctx.on('generateBefore', () => { cache = {}; });
+  ctx.on('generateBefore', () => { cache.reset(); });
 
   return function fragmentCache(id, fn) {
-    if (this.cache && cache[id] != null) return cache[id];
+    if (this.cache && cache.has(id)) return cache.get(id);
 
-    cache[id] = fn();
     const result = fn();
+    cache.set(id, result);
     return result;
   };
 };

package.json

@@ -52,6 +52,7 @@
     "hexo-util": "^1.6.1",
     "js-yaml": "^3.12.0",
     "lodash": "^4.17.11",
+    "lru-cache": "^5.1.1",
     "micromatch": "^4.0.2",
     "moment": "^2.22.2",
     "moment-timezone": "^0.5.21",