feat(wrapper): allow destroy() method to work with functional components

docs/api/options.md

@@ -206,6 +206,9 @@ expect(wrapper.vm.$route).toBeInstanceOf(Object)
 
 Component will be attached to DOM when rendered if set to `true`.
 
+When attaching to the DOM, you should call `wrapper.destroy()` at the end of your test to
+remove the rendered elements from the document and destroy the component instance.
+
 ## attrs
 
 - type: `Object`

docs/api/wrapper/destroy.md

@@ -17,3 +17,8 @@ mount({
 }).destroy()
 expect(spy.calledOnce).toBe(true)
 ```
+
+if `attachToDocument` was set to `true` when mounted, the component DOM elements will
+also be removed from the document.
+
+For functional components, `destroy` only removes the rendered DOM elements from the document.

packages/test-utils/src/wrapper.js

@@ -124,16 +124,22 @@ export default class Wrapper implements BaseWrapper {
    * Calls destroy on vm
    */
   destroy(): void {
-    if (!this.isVueInstance()) {
-      throwError(`wrapper.destroy() can only be called on a Vue instance`)
+    if (!this.isVueInstance() && !this.isFunctionalComponent) {
+      throwError(
+        `wrapper.destroy() can only be called on a Vue instance or ` +
+          `functional component.`
+      )
     }
 
     if (this.element.parentNode) {
       this.element.parentNode.removeChild(this.element)
     }
-    // $FlowIgnore
-    this.vm.$destroy()
-    throwIfInstancesThrew(this.vm)
+
+    if (this.isVueInstance()) {
+      // $FlowIgnore
+      this.vm.$destroy()
+      throwIfInstancesThrew(this.vm)
+    }
   }
 
   /**

test/specs/wrapper/destroy.spec.js

@@ -40,6 +40,21 @@ describeWithShallowAndMount('destroy', mountingMethod => {
     expect(wrapper.vm.$el.parentNode).to.be.null
   })
 
+  it('removes functional component element from document.body', () => {
+    const wrapper = mountingMethod(
+      {
+        functional: true,
+        render: h => {
+          return h('div', {}, [])
+        }
+      },
+      { attachToDocument: true }
+    )
+    expect(wrapper.element.parentNode).to.equal(document.body)
+    wrapper.destroy()
+    expect(wrapper.element.parentNode).to.be.null
+  })
+
   it('throws if component throws during destroy', () => {
     const TestComponent = {
       template: '<div :p="a" />',