How to fix this.state must be initialized in constructor, not in class body in React

Class Fields vs Class Properties:
The official JavaScript specification now includes class fields (ES2022), which is slightly different from the Babel class properties transform proposal. The main difference is initialization timing - class fields are initialized before the constructor runs, while the Babel transform initializes them inside the constructor.

Migration Strategy:
When updating older React projects, you may encounter thousands of components using class field syntax. Instead of converting all components to use constructors, prioritize fixing your build configuration. Modern tooling (Create React App 2.0+, Next.js 9.3+, Vite) includes this support out of the box.

Function Components Alternative:
If you're starting a new project or refactoring, consider using function components with hooks instead of class components. This avoids the entire class initialization debate:

function MyComponent() {
  const [count, setCount] = useState(0);
  const [message, setMessage] = useState('Hello');

  return <div>{count}</div>;
}

Webpack Configuration:
If using custom Webpack setup, ensure babel-loader is properly configured:

module.exports = {
  module: {
    rules: [
      {
        test: /\.jsx?$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: ['@babel/preset-env', '@babel/preset-react'],
            plugins: ['@babel/plugin-proposal-class-properties']
          }
        }
      }
    ]
  }
};

Debugging Tips:
To verify if class properties are working, add a simple test component and check if it compiles. If compilation succeeds but runtime fails, the issue may be in your target browser support rather than build configuration.