What is the difference between constructor and getInitialState?

You should initialize state in the constructor when using ES6 classes, and getInitialState() method when using React.createClass().

Using ES6 classes:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      /* initial state */
    };
  }
}

Using React.createClass():

const MyComponent = React.createClass({
  getInitialState() {
    return {
      /* initial state */
    };
  },
});

Note: React.createClass() is deprecated and removed in React v16. Use plain JavaScript classes instead.