change to more versatile merge behaviour

README.md

@@ -67,6 +67,7 @@
       - [`noscript` ([Object])](#noscript-object)
       - [`changed` (Function)](#changed-function)
     - [How `metaInfo` is Resolved](#how-metainfo-is-resolved)
+        - [Lists of Tags](#lists-of-tags)
 - [Performance](#performance)
     - [How to prevent the update on the initial page render](#how-to-prevent-the-update-on-the-initial-page-render)
 - [FAQ](#faq)
@@ -517,6 +518,69 @@ You can define a `metaInfo` property on any component in the tree. Child compone
 
 If both `<parent>` _and_ `<child>` define a `title` property inside `metaInfo`, then the `title` that gets rendered will resolve to the `title` defined inside `<child>`.
 
+#### Lists of Tags
+
+When specifying an array in `metaInfo`, like in the below examples, the default behaviour is to simply concatenate the lists.
+
+**Input:**
+```js
+// parent component
+{
+  metaInfo: {
+    meta: [
+      { charset: 'utf-8' },
+      { name: 'description', content: 'foo' }
+    ]
+  }
+}
+// child component
+{
+  metaInfo: {
+    meta: [
+      { name: 'description', content: 'bar' }
+    ]
+  }
+}
+```
+
+**Output:**
+```html
+<meta charset="utf-8">
+<meta name="description" content="foo">
+<meta name="description" content="bar">
+```
+
+This is not what we want, since the meta `description` needs to be unique for every page. If you want to change this behaviour such that `description` is instead replaced, then give it a `vmid`:
+
+**Input:**
+```js
+// parent component
+{
+  metaInfo: {
+    meta: [
+      { charset: 'utf-8' },
+      { vmid: 'description', name: 'description', content: 'foo' }
+    ]
+  }
+}
+// child component
+{
+  metaInfo: {
+    meta: [
+      { vmid: 'description', name: 'description', content: 'bar' }
+    ]
+  }
+}
+```
+
+**Output:**
+```html
+<meta charset="utf-8">
+<meta vmid="description" name="description" content="bar">
+```
+
+While solutions like `react-helmet` manage the occurrence order and merge behaviour for you automatically, it involves a lot more code and is therefore prone to failure in some edge-cases, whereas this method is _almost_ bulletproof because of its versatility.
+
 # Performance
 
 On the client, `vue-meta` batches DOM updates using [`requestAnimationFrame`](https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame). It needs to do this because it registers a Vue mixin that subscribes to the [`beforeMount`](https://vuejs.org/api/#beforeMount) lifecycle hook on all components in order to be notified that renders have occurred and data is ready. If `vue-meta` did not batch updates, the DOM meta info would be re-calculated and re-updated for every component on the page in quick-succession.

src/shared/getComponentOption.js

@@ -34,14 +34,22 @@ export default function getComponentOption (opts, result = {}) {
       }
 
       // merge with existing options
-      result = deepmerge(result, data, { arrayMerge })
+      result = deepmerge(result, data, {
+        clone: true,
+        arrayMerge
+      })
 
       // collect & aggregate child options if deep = true
       if (deep) {
         const { $children } = component
         for (let i = 0, len = $children.length; i < len; i++) {
           const component = $children[i]
-          result = getComponentOption({ option, deep, component, arrayMerge }, result)
+          result = getComponentOption({
+            option,
+            deep,
+            component,
+            arrayMerge
+          }, result)
         }
       }
 

src/shared/getMetaInfo.js

@@ -9,40 +9,29 @@ import getComponentOption from './getComponentOption'
  */
 export default function getMetaInfo (component) {
   // collect & aggregate all metaInfo $options
-  const info = getComponentOption({
+  let info = getComponentOption({
     component,
     option: 'metaInfo',
     deep: true,
-    // In order to prevent certain tags from being overwritten,
+    arrayMerge (target, source) {
-    // (like <meta name="description" ...> being overwritten by
+      const destination = []
-    // <meta name="keywords" ...>), we need to specify a different
+      for (let targetIndex in target) {
-    // array merge strategy. This strategy exploits a trick
+        const targetItem = target[targetIndex]
-    // with associative arrays in JS using O(1) lookup
+        let shared = false
-
+        for (let sourceIndex in source) {
-    /* eslint-disable no-labels */
+          const sourceItem = source[sourceIndex]
-    arrayMerge (oldTags, newTags) {
+          if (targetItem.vmid === sourceItem.vmid) {
-      const updatedTags = []
+            shared = true
-      for (let oldTagIndex in oldTags) {
+            break
-        const oldTag = oldTags[oldTagIndex]
-        let sharedAttributes = false
-        ifTagsHaveEqualSharedAttributeValues: for (let newTagIndex in newTags) {
-          const newTag = newTags[newTagIndex]
-          for (let attribute in newTag) {
-            if (newTag.hasOwnProperty(attribute) && oldTag.hasOwnProperty(attribute)) {
-              if (oldTag[attribute] === newTag[attribute]) {
-                sharedAttributes = true
-                break ifTagsHaveEqualSharedAttributeValues
-              }
-            }
           }
         }
-        if (!sharedAttributes) {
+        if (!shared) {
-          updatedTags.push(oldTag)
+          destination.push(targetItem)
         }
       }
-      return updatedTags.concat(newTags)
+
+      return destination.concat(source)
     }
-    /* eslint-enable no-labels */
   })
 
   // if any info options are a function, coerce them to the result of a call