What is the purpose of `getSnapshotBeforeUpdate()` lifecycle method?

The new getSnapshotBeforeUpdate() lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to componentDidUpdate().

class MyComponent extends React.Component {
  getSnapshotBeforeUpdate(prevProps, prevState) {
    // ...
  }
}

This lifecycle method along with componentDidUpdate() covers all the use cases of componentWillUpdate().