fix($seo): Avoid duplicate description meta at runtime. (close: #665)

Author: ulivz

lib/app/root-mixins/updateMeta.js

@@ -6,32 +6,43 @@ export default {
       this.$ssrContext.description = this.$page.description || this.$description
     }
   },
+
   mounted () {
     // update title / meta tags
-    this.currentMetaTags = []
+    this.currentMetaTags = new Set()
+
     const updateMeta = () => {
       document.title = this.$title
       document.documentElement.lang = this.$lang
-      const meta = [
-        {
-          name: 'description',
-          content: this.$description
-        },
-        ...(this.$page.frontmatter.meta || [])
-      ]
-      this.currentMetaTags = updateMetaTags(meta, this.currentMetaTags)
+      const userMeta = this.$page.frontmatter.meta || []
+      const meta = userMeta.slice(0)
+      const useGlobalDescription = userMeta.filter(m => m.name === 'description').length === 0
+
+      // #665 Avoid duplicate description meta at runtime.
+      if (useGlobalDescription) {
+        meta.push({ name: 'description', content: this.$description })
+      }
+
+      // Including description meta coming from SSR.
+      const descriptionMetas = document.querySelectorAll('meta[name="description"]')
+      if (descriptionMetas.length) {
+        descriptionMetas.forEach(m => this.currentMetaTags.add(m))
+      }
+
+      this.currentMetaTags = new Set(updateMetaTags(meta, this.currentMetaTags))
     }
     this.$watch('$page', updateMeta)
     updateMeta()
   },
+
   beforeDestroy () {
     updateMetaTags(null, this.currentMetaTags)
   }
 }
 
 function updateMetaTags (meta, current) {
   if (current) {
-    current.forEach(c => {
+    [...current].forEach(c => {
       document.head.removeChild(c)
     })
   }