In React, the componentWillUnmount() lifecycle method is called just before a component is unmounted from the DOM. This method is useful for cleaning up any resources or state changes that have been made in the component.

Here is an example of how componentWillUnmount could be used in a React component:

class MyComponent extends React.Component {
  componentWillUnmount() {
    // Clean up any resources or 
    // state changes here

    // For example, you could cancel
    // any pending network requests
    if (this.pendingRequest) {
      this.pendingRequest.cancel();
    }

    // Or, you could remove any event
    //listeners that were added to the component
    if (this.eventListener) {
      this.eventListener.remove();
    }
  }

  render() {
    return <div>My component</div>;
  }
}

In this example, the componentWillUnmount method cancels any pending network requests and removes any event listeners that were added to the component.

Here is another example of how componentWillUnmount could be used in a React component:

class MyComponent extends React.Component {
  componentWillUnmount() {
    // Clean up any resources or state changes here

    // For example, you could clear any timers that were set in the component
    if (this.timer) {
      clearInterval(this.timer);
    }

    // Or, you could save any data to local storage
    if (this.state.data) {
      localStorage.setItem('data', this.state.data);
    }
  }

  render() {
    return <div>My component</div>;
  }
}

In this example, the componentWillUnmount method clears any timers that were set in the component and saves any data to local storage.

It's important to note that this method is called just before the component is unmounted, not when it is re-rendered. If you need to perform any cleanup when a component is re-rendered, you should use the componentDidUpdate lifecycle method instead.

Additionally, it's recommended to avoid performing any complex logic or operations in this method, as it can negatively impact the performance of your application. This method should be used only for simple cleanup tasks.