free-soul free-soul - 6 days ago 6
Javascript Question

constructor vs componentWillMount; what a componentWillMount can do that a constructor cannot?

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
?

leo leo
Answer

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?

No, see here.

The following code doesn't block render (bear in mind this would be an anti pattern anyways to call setState there)

componentWillMount: function() {
     new Promise((resolve, reject) => {
        setTimeout(()=> {
            resolve();
        }, 2000)
     }).then(() => this.setState({ promiseResult: 'World' }));
  },

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

No, for ES6 classes you can discard componentWillMount. It is only needed if you use React.createClass({... })

EDIT: Apparently, I'm wrong. Thanks to @Swapnil for pointing this out. Here is the discussion.

React throws a warning if there is a side effect in the constructor which modifies state in another component, because it assumes that setState in the constructor itself and potentially during render() is being called. So no side effects in the constructor are desired.

This is not the case if you do it in componentWillMount, no errors are thrown. On the other hand, the guys from facebook discourage side effects in componentWillMount also. So if you don't have side any effects, you could use the constructor instead of componentWillMount. For side effects it is recommended to use componentDidMount instead of componentWillMount. Either way, you don't need componentWillMount.

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