Are there cases when jQuery and React can be used together on the same page?

Yes, they exist.

This can happen when:

• Gradually integrating React into an existing jQuery project. Part of the functionality is rewritten in React, while the old jQuery code continues to work.
• Using old jQuery plugins or libraries. If there are no React equivalents or their integration is complex, jQuery may need to be retained.
• Working with third-party widgets or components. Some third-party tools may depend on jQuery.
• Minor DOM manipulations. For simple tasks that do not require a full React component, it is sometimes faster and easier to use jQuery.

Example of interaction:

// React code
class MyComponent extends React.Component {
  componentDidMount() {
    // Using jQuery to manipulate DOM outside React tree
    $('#someButton').on('click', () => {
      alert('Button clicked via jQuery!');
    });
  }

  render() {
    return <button id="someButton">Click me</button>;
  }
}

// Initializing React component
ReactDOM.render(<MyComponent />, document.getElementById('root'));

When using this approach, it is important to consider:

• Separation of concerns: React is responsible for managing state and rendering, while jQuery is used for DOM manipulations not managed by React.
• Performance: Mixing can lead to suboptimal performance and debugging difficulties.
• Compatibility: Ensure that library versions are compatible.

Using both libraries should be a conscious decision, often temporary or due to necessity. It is preferable to migrate to pure React where possible.