docs(web-components): implementation notes for React integration (#5000)

### Related Ticket(s)

Refs #4975.

### Description

Add notes on how React integration for `@carbon/ibmdotcom-web-components` works, and how build procedure automates in this area.

### Changelog

**New**

- Notes on how React integration for `@carbon/ibmdotcom-web-components` works, and how build procedure automates in this area.

packages/web-components/IMPLEMENTATION_NOTES.md

@@ -16,6 +16,11 @@
   - [Composite components](#composite-components)
   - [Components rendering modal](#components-rendering-modal)
 - [CTA components](#cta-components)
+- [React integration](#react-integration)
+  - [React wrapper generator](#react-wrapper-generator)
+  - [Build procedure to generate React wrapper](#build-procedure-to-generate-react-wrapper)
+    - [Limited components to generate React wrapper](#limited-components-to-generate-react-wrapper)
+  - [Non-React APIs for React integration](#non-react-apis-for-react-integration)
 - [Container components](#container-components)
   - [Triggering action dispatcher](#triggering-action-dispatcher)
 - [Masthead search](#masthead-search)
@@ -114,6 +119,69 @@ There are some common behaviors in CTA components, that is implemented by [`CTAM
 - Send an event (`dds-cta-request-video-data`) when user clicks on CTA so that the lightbox video player code can launch the light box
 - (Video caption/duration/thumbnail)
 
+## React integration
+
+While Angular/Vue require limited/no change in application to use Web Components, React has some problems with Web Components support. React heavily relies on its knowledge with regard to how intrinsic HTML elements work.
+
+For example, if you have `<dds-foo custom-boolean={true}>`, the DOM becomes `<dds-foo custom-boolean="true">` that is different from how `<button disabled={true}>` is rendered to DOM, which is, `<button disabled>` (empty string in `disabled` attribute value).
+
+Another example is `<dds-foo onFoo={handleFoo}>` that doesn't attach an event handler, whereas `<div onClick={handleClick}>` does.
+
+### React wrapper generator
+
+Web Components community [made React core team aware of above issue](https://github.com/facebook/react/issues/11347), and created a [proposal](https://github.com/reactjs/rfcs/pull/15) to address that.
+
+`carbon-web-components` library implements a variant of the proposal, with [`createReactCustomElementType()` function](https://github.com/carbon-design-system/carbon-web-components/blob/v1.11.0/src/globals/wrappers/createReactCustomElementType.ts). Below example create a React component, `BXDropdown`, from `<bx-dropdown>` custom element:
+
+```javascript
+import createCustomElementType, { booleanSerializer } from 'carbon-web-components/es/globals/wrappers/createCustomElementType';
+
+const BXDropdown = createCustomElementType('bx-dropdown', {
+  disabled: {
+    serialize: booleanSerializer,
+  },
+  helperText: {
+    attribute: 'helper-text',
+  },
+  onBeforeSelect: {
+    event: 'bx-dropdown-beingselected',
+  },
+});
+```
+
+We call React components that `createCustomElementType()` creates, e.g. `<BXDropdown>`, _React wrappers_. React wrapper provides the following enhancement over directly using `<bx-dropdown>` in JSX:
+
+- `<BXDropdown disabled={true}>` yeilds to `<bx-dropdown disabled>` and `<BXDropdown disabled={false}>` causes `disabled` attribute removed from `<bx-dropdown>`.
+- `<BXDropdown helperText="The helper text">` yields to `<bx-dropdown helper-text="The helper text">`.
+- `<BXDropdown onBeforeSelect={handleBeforeSelect}>` attaches `bx-dropdown-beingselected` event to `<bx-dropdown>`.
+
+### Build procedure to generate React wrapper
+
+`carbon-web-components` as well as `@carbon/ibmdotcom-web-components` have a build procedure to generate the React wrapper using `createReactCustomElementType`. It uses a [Babel plugin](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/tools/babel-plugin-create-react-custom-element-type.js) that transforms the original custom element type code to a corresponding React wrapper, by looking at `lit-element`'s `@property` decorator and `eventXXX` static properties in the custom element class. The build procedure can be found at [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/gulp-tasks/build.js#L159-L170).
+
+There is [another Babel plugin](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/tools/babel-plugin-create-react-custom-element-type-def.js) that transforms the original custom element type code to a corresponding TypeScript definition to above React wrapper, leveraging the same `@property` and `eventXXX` in the custom element class. The build procedure can be found at [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/gulp-tasks/build.js#L385-L397).
+
+#### Limited components to generate React wrapper
+
+At the point of `v1.15.0` release, we decided that we generate React wrappers only for components without pure-React counterpart, for example, `<dds-carousel>` and `<dds-leaving-ibm-modal>`.
+
+Those kind of components should have `/* @__GENERATE_REACT_CUSTOM_ELEMENT_TYPE__ */` annotation at the default export. Such annotation is read by a [Babel plugin](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/tools/babel-plugin-scan-create-react-custom-element-type-candidates.js) so the build procedure can [harvest the list of files to generate React wrappers for](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/gulp-tasks/build.js#L126-L148).
+
+### Non-React APIs for React integration
+
+There are a couple of types of non-React APIs that application can use with React integration code:
+
+- Constant/enum definitions, e.g. `PICTOGRAM_PLACEMENT` for `<dds-card>`
+- `mapStateToProps()`/`mapDispatchToProps()` for Redux integration
+
+Because of non-React nature, the implementation is identical to the original Web Components code, while we want to make them available to React users. Therefore, we create modules in `es/components-react` directory that proxies to one in `es/components`, like:
+
+```javascript
+export * from '../../components/card/defs.js';
+```
+
+The build procedure can be found at [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0-rc.1/packages/web-components/gulp-tasks/build.js#L257-L282).
+
 ## Container components
 
 Container components are inheritances of composite components that connects to `@carbon/ibmdotcom-service`. Connecting to `@carbon/ibmdotcom-service` is done via a Redux store, [`@carbon/ibmdotcom-services-store`](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/tree/v1.13.0/packages/services-store). Connecting to `@carbon/ibmdotcom-services-store` is done by [`ConnectMixin`](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/globals/mixins/connect.ts) that has a similar feature set to [`react-redux`](https://react-redux.js.org).