feat: add hook for transforming h's arguments (#851)

Note: this is for internal use (e.g. `@vue/test-utils`) only

packages/runtime-core/__tests__/h.spec.ts

@@ -1,9 +1,11 @@
-import { h } from '../src/h'
+import { h, transformHArgs, resetTransformHArgs } from '../src/h'
 import { createVNode } from '../src/vnode'
+import { ComponentInternalInstance } from '@vue/runtime-core'
+import { createApp } from '@vue/runtime-dom'
 
 // Since h is a thin layer on top of createVNode, we are only testing its
 // own logic here. Details of vnode creation is tested in vnode.spec.ts.
-describe('renderer: h', () => {
+const testH = () => {
   test('type only', () => {
     expect(h('div')).toMatchObject(createVNode('div'))
   })
@@ -57,4 +59,62 @@ describe('renderer: h', () => {
       })
     )
   })
+}
+
+describe('renderer: h', testH)
+
+describe('renderer: transformHArgs', () => {
+  describe('no-op pass-through', () => {
+    beforeAll(() => {
+      transformHArgs((hArgs: unknown[]) => hArgs)
+    })
+ 
+    afterAll(resetTransformHArgs)
+ 
+    testH()
+  })
+
+  describe('args is used directly, without merging', () => {
+    beforeAll(() => {
+      transformHArgs(() => ['h1', 'Hello World'])
+    })
+ 
+    afterAll(resetTransformHArgs)
+ 
+    test('nodes become an h1 with text inside', () => {
+      expect(h('div')).toMatchObject(createVNode('h1', null, 'Hello World'))
+    })
+
+    test('resetting transformHArgs turns things back to normal', () => {
+      expect(h('div')).toMatchObject(createVNode('h1', null, 'Hello World'))
+      resetTransformHArgs()
+      expect(h('div')).toMatchObject(createVNode('div'))
+    })
+  })
+
+  test('receives component instance as the 2nd arg', () => {
+    transformHArgs((_: unknown[], instance: ComponentInternalInstance) => {
+      return ['h1', instance.type.name] // <h1>{{ name }}</h1>
+    })
+
+    const vm = createApp({
+      // this will be the name of the component in the h1
+      name: 'Root Component',
+      render() {
+        return h({
+          // this code will never execute,
+          // because it is overridden by the transformHArgs method
+          render() {
+            return h('h2', 'Stub Text')
+          }
+        })
+      }
+    })
+
+    // we need to mount everything so that the instance passed to
+    // transformHArgs isn't null
+    vm.mount('body')
+
+    expect(document.body.outerHTML).toContain('<h1>Root Component</h1>')
+  })
 })