docs: note about createElement workaround

Author: antfu

README.md

@@ -401,6 +401,37 @@ app2.component('Bar', Bar) // equivalent to Vue.use('Bar', Bar)
 
 </details>
 
+### `createElement` / `h`
+
+<summary>
+⚠️ <code>createElement</code> / <code>h</code> workaround
+</summary>
+
+<br>
+
+`createElement` / `h` in Vue 2 is only accessable in `render()` function. To use it outside of `render()`, you can explicitly bind a component instance to it.
+
+> :warning: **Warning**: This ability is provided as a workaround Vue 2, it's not part of the Vue 3 API.
+
+```jsx
+import { h as _h } from '@vue/composition-api'
+
+export default {
+  setup() {
+    const vm = getCurrentInstance()
+    const h = _h.bind(vm)
+
+    return () =>
+      h('div', {
+        ref: 'root',
+      })
+  },
+}
+```
+
+</details>
+
+
 ### `shallowReadonly`
 
 <details>

src/apis/createElement.ts

@@ -11,7 +11,7 @@ import { VNode, VNodeChildren, VNodeData } from 'vue/types/vnode'
 
 export interface H extends CreateElement {
   (
-    this: ComponentInternalInstance | null,
+    this: ComponentInternalInstance | null | undefined,
     tag?:
       | string
       | Component<any, any, any, any>
@@ -20,7 +20,7 @@ export interface H extends CreateElement {
     children?: VNodeChildren
   ): VNode
   (
-    this: ComponentInternalInstance | null,
+    this: ComponentInternalInstance | null | undefined,
     tag?:
       | string
       | Component<any, any, any, any>
@@ -33,8 +33,8 @@ export interface H extends CreateElement {
 
 let fallbackCreateElement: CreateElement
 
-export const createElement = function createElement(this, ...args: any) {
-  const instance = this ? this.proxy : getCurrentInstance()?.proxy
+export const createElement: H = function createElement(this, ...args: any) {
+  const instance = this?.proxy || getCurrentInstance()?.proxy
   if (!instance) {
     __DEV__ &&
       warn('`createElement()` has been called outside of render function.')
@@ -48,4 +48,4 @@ export const createElement = function createElement(this, ...args: any) {
   }
 
   return instance.$createElement.apply(instance, args)
-} as H
+}