Why use componentWillMount and not componentDidMount? From the docs:
> componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state synchronously in this method will not trigger a re-rendering. Avoid introducing any side-effects or subscriptions in this method.
> componentDidMount() is invoked immediately after a component is mounted. Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request. Setting state in this method will trigger a re-rendering.
> componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state synchronously in this method will not trigger a re-rendering. Avoid introducing any side-effects or subscriptions in this method.
> componentDidMount() is invoked immediately after a component is mounted. Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request. Setting state in this method will trigger a re-rendering.