As far as I could see, the only thing a componentWillMount can do and a constructor cannot is to call setState.

componentWillMount() {

    setState({ isLoaded: false });

}

Since we have not called render yet, a setState in componentWillMount will prepare the state object before we enter the first render() pass. Which is essentially the same thing a constructor does:

constructor(props) {

    super(props);

    this.state = { isLoaded: false };

}

But I see another use case where componentWillMount is useful (on server side).

Let's consider something asynchronous:

componentWillMount() {

    myAsyncMethod(params, (result) => {

        this.setState({ data: result });

    })

}

Here we cannot use the constructor as assignment to this.state won't trigger render().

What about setState in componentWillMount? According to React docs:

componentWillMount() is invoked immediately before mounting occurs. It
is called before render(), therefore setting state in this method will
not trigger a re-rendering. Avoid introducing any side-effects or
subscriptions in this method.

So, here I think React will use the new state value for the first render and avoids a re-render.

Question 1: Does this means, inside componentWillMount, if we call setState in an async method's callback (can be a promise callback), React blocks initial rendering until the callback is executed?

Having this setup on client-side (yes I see that use case in server-side rendering), if I assume the above is true, I will not see anything until my asynchronous method completes.

Am I missing any concepts?

Question 2: Are the any other use cases that I can achieve with componentWillMount only, but not using the constructor and componentDidMount?