fix: warn if property could not be overwritten by mocks

Author: eddyerburgh

docs/en/guides/common-tips.md

@@ -109,6 +109,8 @@ mount(Component, {
 })
 ```
 
+**Note some plugins, like Vue Router, add read-only properties to the global Vue constructor. This makes it impossible to reinstall the plugin on a localVue constructor, or add mocks for these properties**
+
 ## Mocking Injections
 
 Another strategy for injected props is simply mocking them. You can do that with the `mocks` option:

src/lib/add-mocks.js

@@ -1,9 +1,14 @@
 // @flow
 import $$Vue from 'vue'
+import { warn } from './util'
 
 export default function addMocks (mockedProperties: Object, Vue: Component) {
   Object.keys(mockedProperties).forEach((key) => {
-    Vue.prototype[key] = mockedProperties[key]
+    try {
+      Vue.prototype[key] = mockedProperties[key]
+    } catch (e) {
+      warn('could not overwrite property $store, this usually caused by a plugin that has added the property as a read-only value')
+    }
     $$Vue.util.defineReactive(Vue, key, mockedProperties[key])
   })
 }

test/unit/specs/mount/options/mocks.spec.js

@@ -84,4 +84,22 @@ describe('mount.mocks', () => {
     const freshWrapper = mount(Component)
     expect(typeof freshWrapper.vm.$store).to.equal('undefined')
   })
+
+  it('logs that a property cannot be overwritten if there are problems writing', () => {
+    const error = sinon.stub(console, 'error')
+    const localVue = createLocalVue()
+    Object.defineProperty(localVue.prototype, '$store', {
+      value: 42
+    })
+    const $store = 64
+    mount(Component, {
+      localVue,
+      mocks: {
+        $store
+      }
+    })
+    const msg = '[vue-test-utils]: could not overwrite property $store, this usually caused by a plugin that has added the property as a read-only value'
+    expect(error.calledWith(msg)).to.equal(true)
+    error.restore()
+  })
 })