title: how to mock and test this.$parents in vue using vue-test-utils
body: <p>I am trying to test a vue component that uses this.$parents.props in it.</p>
  
  <p>e.g.</p>
  
  <pre><code>export default class item extends Vue {
    private point = this.$parent.$props.point;
  }
  </code></pre>
  
  <p>The reason why I think I need to mock this.$parents.$props is because I get an error like this.</p>
  
  <pre><code>TypeError: Cannot read property 'point' of undefined
  </code></pre>
  
  <p>I have tried mounting options, parentComponent, but it throws an error saying "[vue-test-utils]: options.parentComponent should be a valid Vue component options object".</p>
  
  <p>This is the code I used for testing.</p>
  
  <pre><code>import Parent from "@/components/Parent.vue";
  
  let wrapper: any;
  describe("item.vue", () =&gt; {
    it("item vue testing", () =&gt; {
      wrapper = mount(item, {
          propsData: {
              num: 1,
          },
          parentComponent: Parent
        });
  
    });
  });
  </code></pre>
  
  <p>What should I do to mock this.$parent.$props? What is the mistake in the above test code?</p>