Ao usar a versão ES6 para definir um Componente React,

NÃO

class A extends React.Component {
  myFunction() {
    // you'd want to use `this` here
  }
  componentDidMount /* or some react component lifecycle method */() {
    // Never do .bind(this) at the place of subscription
    SomeEvent.subscribe(this.myFunction.bind(this));
  }
  componentWillUnmount() {
    // because, now this will not work
    SomeEvent.unsubscribe(this.myFunction.bind(this));
  }
}

FAZ

class B extends React.Component {
  constructor(...args) {
    super(...args);
    // bind it and make it an instance method instead of prototype method
    this.myFunction = this.myFunction.bind(this);
  }
  myFunction() { /* this */ }
  componentDidMount() {
    SomeEvent.subscribe(this.myFunction);
  }
  componentWillUnmount() {
    SomeEvent.unsubscribe(this.myFunction);
  }
}

MAS

const C = React.createClass({
  //- - - - - - ^ - the magic happens here
  myFunction() {
    // this
  },
  componentDidMount() {
    SomeEvent.subscribe(this.myFunction);
  },
  componentWillUnmount() {
    SomeEvent.unsubscribe(this.myFunction);
  }
});

Inspirado neste artigo – [ https://www.webreflection.co.uk/blog/2015/10/22/how-to-add-dom-events-listeners ] por
@WebReflection

Isso também é mencionado nos documentos oficiais do React – [ https://facebook.github.io/react/blog/2015/01/27/react-v0.13.0-beta-1.html#autobinding ].