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# react-lifecycles-compat
## What is this project?
React version 17 will deprecate several of the class component API lifecycles: `componentWillMount`, `componentWillReceiveProps`, and `componentWillUpdate`. (Read the [Update on Async rendering blog post](https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html) to learn more about why.) A couple of new lifecycles are also being added to better support [async rendering mode](https://reactjs.org/blog/2018/03/01/sneak-peek-beyond-react-16.html).
Typically, this type of change would require third party libraries to release a new major version in order to adhere to semver. However, the `react-lifecycles-compat` polyfill offers a way to use the new lifecycles with older versions of React as well (0.14.9+) so no breaking release is required. This enables shared libraries to support both older and newer versions of React simultaneously.
## How can I use the polyfill
First, install the polyfill from NPM:
```sh
# Yarn
yarn add react-lifecycles-compat
# NPM
npm install react-lifecycles-compat --save
```
Next, update your component and replace any of the deprecated lifecycles with new ones introduced with React 16.3. (Refer to the React docs for [examples of how to use the new lifecycles](https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html).)
Lastly, use the polyfill to make the new lifecycles work with older versions of React:
```js
import React from 'react';
import {polyfill} from 'react-lifecycles-compat';
class ExampleComponent extends React.Component {
static getDerivedStateFromProps(nextProps, prevState) {
// Normally this method would only work for React 16.3 and newer,
// But the polyfill will make it work for older versions also!
}
getSnapshotBeforeUpdate(prevProps, prevState) {
// Normally this method would only work for React 16.3 and newer,
// But the polyfill will make it work for older versions also!
}
// render() and other methods ...
}
// Polyfill your component so the new lifecycles will work with older versions of React:
polyfill(ExampleComponent);
export default ExampleComponent;
```
## Which lifecycles are supported?
Currently, this polyfill supports [static `getDerivedStateFromProps`](https://reactjs.org/docs/react-component.html#static-getderivedstatefromprops) and [`getSnapshotBeforeUpdate`](https://reactjs.org/docs/react-component.html#getsnapshotbeforeupdate)- both introduced in version 16.3.
## Validation
Note that in order for the polyfill to work, none of the following lifecycles can be defined by your component: `componentWillMount`, `componentWillReceiveProps`, or `componentWillUpdate`.
Note also that if your component contains `getSnapshotBeforeUpdate`, `componentDidUpdate` must be defined as well.
An error will be thrown if any of the above conditions are not met.

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