Fixing error: Warning: getInitialState was defined on a plain JavaScript class. This is only supported for classes created using React.createClass.

Let’s say you upgrade an old React component, like so:

class MyComponent extends React.Component {
  constructor() {
    super();
  }

  getInitialState() {
    return { test: 1 }
  }

  render() {
    render (
Hello World!
);
  }
}

When you go to use this, you will get this error:

Warning: getInitialState was defined on a plain JavaScript class. 
This is only supported for classes created using React.createClass.

React can apparently detect the difference between objects and new ES6 classes, and wants you to use a more conventional class construction technique.

The correct answer is to do what the error says, and move the contents of the getInitialState function into the constructor:

class MyComponent extends React.Component {
  constructor() {
    super();

    this.state = { test: 1 };
  }

  render() {
    render (
Hello World!
);
  }
}