# @babel/plugin-transform-react-constant-elements

> Treat React JSX elements as value types and hoist them to the highest scope.

This plugin can speed up reconciliation and reduce garbage collection pressure by hoisting
React elements to the highest possible scope, preventing multiple unnecessary reinstantiations.

## Example

**In**

```jsx
const Hr = () => {
  return <hr className="hr" />;
};
```

**Out**

```jsx
const _ref = <hr className="hr" />;

const Hr = () => {
  return _ref;
};
```

**Deopts**

- **Spread Operator**

  ```jsx
  <div {...foobar} />
  ```

- **Refs**

  ```jsx
  <div ref="foobar" />
  <div ref={node => this.node = node} />
  ```

- **Mutable Properties**

> See https://github.com/facebook/react/issues/3226 for more on this

  ```js
  <div width={{width: 100}} />
  ```

## Installation

```sh
npm install --save-dev @babel/plugin-transform-react-constant-elements
```

## Usage

### Via `.babelrc` (Recommended)

**.babelrc**

```json
{
  "plugins": ["@babel/transform-react-constant-elements"]
}
```

## Options

### `allowMutablePropsOnTags`

`Array<string>`, defaults to `[]`

If you are using a particular library (like react-intl) that uses object properties, and you are sure
that the element won't modify its own props, you can whitelist the element so that objects are allowed.

This will skip the `Mutable Properties` deopt.

```json
{
  "plugins": [
    ["@babel/transform-react-constant-elements", {"allowMutablePropsOnTags": ["FormattedMessage"]}],
  ]
}

```

### Via CLI

```sh
babel --plugins @babel/transform-react-constant-elements script.js
```

### Via Node API

```javascript
require("@babel/core").transform("code", {
  plugins: ["@babel/transform-react-constant-elements"]
});
```

## References

* [[facebook/react#3226] Optimizing Compiler: Reuse Constant Value Types like ReactElement](https://github.com/facebook/react/issues/3226)