Warning: you are calling reactdomclient.createroot() on a container that has already been passed to createroot() before. instead, call root.render() on the existing root instead if you want to update it.

When you see the warning “warning: you are calling reactdomclient.createroot() on a container that has already been passed to createroot() before. instead, call root.render() on the existing root instead if you want to update it.”, it means that you are attempting to create a new root container using ReactDOM.render() on an element that has already been mounted before.

This warning typically occurs when you try to use ReactDOM.render() multiple times on the same container element. The ReactDOM.render() function is used to render a React element into the DOM, but it can only be called once for each container element.

If you want to update the existing root instead of creating a new one, you should use the root.render() method on the existing root object. This method allows you to update the contents of the root container without creating a new container.

Here’s an example to illustrate this:

    
      <div id="root"></div>

      <script>
        const rootContainer = document.getElementById('root');

        // Initial render
        ReactDOM.render(<App />, rootContainer);

        // Trying to create a new root container
        ReactDOM.render(<NewApp />, rootContainer);
        // This will trigger the warning as the container has already been passed to createroot() before.

        // Instead, update the existing root
        const root = ReactDOM.createRoot(rootContainer);
        root.render(<UpdatedApp />);
        // This will update the contents of the existing root container without creating a new one.
      </script>
    
  

In this example, we first render the initial application component (App) into the root container using ReactDOM.render(). Then, we mistakenly try to create a new root container using ReactDOM.render() again with a different component (NewApp), resulting in the warning.

To resolve the issue, we create a root object using ReactDOM.createRoot() on the same container element. Then, we use the root.render() method to update the contents of the root container with the desired component (UpdatedApp). This way, we avoid the warning and successfully update the existing root.