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.