change meta tags merge function

Author: taichunmin

Diff for: packages/@vuepress/core/lib/client/root-mixins/updateMeta.js

@@ -1,19 +1,4 @@
-import unionBy from 'lodash/unionBy'
-
-// @see https://ogp.me/
-const META_TYPE_ARRAY = [
-  'article:author',
-  'article:tag',
-  'book:author',
-  'book:tag',
-  'music:album',
-  'music:musician',
-  'og:locale:alternate',
-  'video:actor',
-  'video:director',
-  'video:tag',
-  'video:writer'
-]
+import transform from 'lodash/transform'
 
 export default {
   // created will be called on both client and ssr
@@ -49,11 +34,18 @@ export default {
     },
 
     getMergedMetaTags () {
-      const pageMeta = this.$page.frontmatter.meta || []
-      // pageMetaTags have higher priority than siteMetaTags
-      // description needs special attention as it has too many entries
-      return unionBy([{ name: 'description', content: this.$description }],
-        pageMeta, this.siteMeta, metaIdentifier)
+      const exists = {}
+      return transform([
+        this.$page.frontmatter.meta || [], // page meta
+        [{ name: 'description', content: this.$description }], // meta description
+        this.siteMeta // site meta
+      ], (merged, meta) => {
+        const filtered = meta.filter(tag => !exists[metaIdentifier(tag)])
+        merged.push(...filtered)
+        filtered.forEach(tag => {
+          exists[metaIdentifier(tag)] = 1
+        })
+      }, [])
     }
   },
 
@@ -101,11 +93,7 @@ function updateMetaTags (newMetaTags, currentMetaTags) {
  */
 function metaIdentifier (tag) {
   for (const item of ['name', 'property', 'itemprop']) {
-    if (tag.hasOwnProperty(item)) {
-      const key = tag[item]
-      const value = tag.hasOwnProperty('content') && ~META_TYPE_ARRAY.indexOf(key) ? tag.content : ''
-      return key + item + value
-    }
+    if (tag.hasOwnProperty(item)) return tag[item] + item
   }
   return JSON.stringify(tag)
 }