Why Does My React 19 HMR Reload the Page and Show Errors in a Hello-World App?

Below is an explanation of what’s likely happening and how you can fix both issues.

What’s Happening

1. Before Reload Error Message
   The error you’re seeing just before the full reload happens because the module update isn’t “hot‐accepted” properly. Without a proper setup for React’s fast refresh, Webpack’s HMR falls back to a full reload when it cannot update the module in place. Your custom module.hot.accept code isn’t sufficient on its own for React 18+/createRoot setups.

2. Full Reload Instead of In‑Place Updates
   When using the new React 18 API (i.e. createRoot), you need to adopt a React Refresh–based workflow. Without it, even trivial updates (like in App) won’t get updated in place but cause a full reload, and you’ll see error messages (often in the browser console) before the reload.

The Fix: Enable React Fast Refresh

React Fast Refresh is the recommended way to set up HMR for React components. It makes the HMR updates seamless without reloading the whole page and helps avoid those transient errors. Here’s what you need to do:

1. Install the Required Dependencies

   Install the React Refresh plugin and related package as development dependencies:

   BASHCopied
   npm install --save-dev @pmmmwh/react-refresh-webpack-plugin react-refresh

2. Update Your Babel Configuration

   In your Babel configuration (for example, in your .babelrc or babel config section in package.json), add the React Refresh Babel plugin. For example, if you have a .babelrc:

   JSONCopied
   {
     "presets": ["@babel/preset-env", "@babel/preset-react"],
     "plugins": [
       // Enable this plugin only during development:
       "react-refresh/babel"
     ]
   }

   (You may want to conditionally add the plugin only in development mode.)

3. Modify Your webpack.config.js

   In your webpack configuration, require and add the React Refresh plugin when in development mode:

   JAVASCRIPTCopied
   const path = require('path');
   const HtmlWebpackPlugin = require('html-webpack-plugin');
   
   // Only require this plugin in development.
   const ReactRefreshWebpackPlugin = require('@pmmmwh/react-refresh-webpack-plugin');
   
   const isDevelopment = process.env.NODE_ENV !== 'production';
   
   module.exports = {
     mode: isDevelopment ? 'development' : 'production',
     entry: './src/index.js',
     output: {
       path: path.resolve(__dirname, 'dist'),
       filename: 'bundle.[contenthash].js',
       clean: true
     },
     devServer: {
       static: {
         directory: './dist',
         publicPath: '/apps/chat-contests/dev'
       },
       port: 1010,
       hot: true,
       allowedHosts: 'all',
       client: {
         webSocketURL: {
           protocol: 'wss',
           port: 443,
           hostname: 'my-hostname.com',
           pathname: '/chat-contests-ws'
         }
       }
     },
     module: {
       rules: [
         {
           test: /\.jsx?$/,
           exclude: /node_modules/,
           use: {
             loader: 'babel-loader',
             options: {
               // If you want to conditionally include the plugin:
               plugins: [
                 isDevelopment && require.resolve('react-refresh/babel')
               ].filter(Boolean)
             }
           }
         },
         {
           test: /\.css$/,
           use: ['style-loader', 'css-loader']
         }
       ]
     },
     resolve: {
       extensions: ['.js', '.jsx']
     },
     plugins: [
       new HtmlWebpackPlugin({
         template: './public/index.html'
       }),
       // Only add ReactRefreshWebpackPlugin in development mode.
       isDevelopment && new ReactRefreshWebpackPlugin()
     ].filter(Boolean)
   };

4. Simplify Your index.js HMR Code

   Once React Fast Refresh is enabled you don’t need to call module.hot.accept manually for your “App” module – Fast Refresh handles that automatically. Your index file can be simplified to:

   JAVASCRIPTCopied
   import React from 'react';
   import { createRoot } from 'react-dom/client';
   import App from './App';
   
   const container = document.getElementById('app');
   const root = createRoot(container);
   
   root.render(<App />);
   
   if (module.hot) {
     module.hot.accept();
   }

   (Note: The module.hot.accept() here is harmless – it serves as a fallback.)

Summary

• The errors occur because your current HMR setup isn’t “aware” of the React 18 createRoot API.
• Setting up React Fast Refresh (via react-refresh and its webpack plugin along with the Babel plugin) will allow HMR to update your components without reloading the whole page.
• Remove custom hot handling logic for individual components and let React Refresh do the heavy lifting.

Implement these changes and you should see that trivial updates (like editing your App component) update in place without errors or a full page reload.