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What is the difference between componentWillMount and componentDidMount in ReactJS?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I looked at Facebook’s documentation at https://facebook.github.io/react/docs/component-specs.html and it mentions how componentWillMount is invoked on the client/server whereas componentDidMount is invoked only on the client. What does componentWillMount do to the server?

Answers:

componentWillMount is essentially the constructor. You can set instance properties that don’t affect render, pull data from a store synchronously and setState with it, and other simple side effect free code you need to run when setting up your component.

It’s rarely needed, and not at all with ES6 classes.

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Answers:

the constructor method is not the same as componentWillMount.

According to the author of Redux, it is risky to dispatch actions from the constructor because it may result in mutating the state while rendering.

However, dispatching from componentWillMount is just fine.

from github issue:

This happens when dispatch() inside one component’s constructor causes a setState() inside another component. React keeps track of the “current owner” for such warnings—and it thinks we’re calling setState() inside the constructor when technically constructor causes a setState() inside some other part of the application.
I don’t think we should handle this—it’s just React trying its best do its job. The solution is, as you correctly noted, to dispatch() inside componentWillMount() instead.

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To add to what FakeRainBrigand said, componentWillMount gets called when rendering React on the server, but componentDidMount is only called on the client.

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componentWillMount is done before the INITIAL render of a component, and is used to assess props and do any extra logic based upon them (usually to also update state), and as such can be performed on the server in order to get the first server side rendered markup.

componentDidMount is performed AFTER the initial render when the DOM has been updated (but crucially BEFORE this DOM update is painted to the browser, allowing you to do all kinds of advanced interactions with the DOM itself). This of course can only happen in the browser itself and so does not occur as part of SSR, as the server can only generate markup and not the DOM itself, this is done after it gets sent to the browser if using SSR

Advanced interactions with the DOM you say? Whaaaat??… Yep – at this point because the DOM has been updated (but the user has not seen the update in browser yet) it is possible to intercept actual painting to the screen by using window.requestAnimationFrame and then do things like measure the actual DOM elements that will be output, to which you can perform further state changes, super useful for example animating to a height of an element that has unknown variable length contents (as you can now measure the contents and assign a height to the animation), or to avoid flash of content scenarios during some state change.

Be very careful though to guard state changes in any componentDid... as otherwise can cause an infinite loop because a state change will also cause a re-render, and hence another componentDid... and on and on and on

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As per the documentation ( https://facebook.github.io/react/docs/react-component.html )

Methods prefixed with will are called right before something happens
and

Methods prefixed with did are called right after something happens.

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componentWillMount https://daveceddia.com/where-fetch-data-componentwillmount-vs-componentdidmount/

There’s a “gotcha,” though: An asynchronous call to fetch data will
not return before the render happens. This means the component will
render with empty data at least once.

There is no way to “pause” rendering to wait for data to arrive. You
cannot return a promise from componentWillMount or wrangle in a
setTimeout somehow.

https://developmentarc.gitbooks.io/react-indepth/content/life_cycle/birth/premounting_with_componentwillmount.html

our Component will not have access to the Native UI (DOM, etc.). We also will not have access to the children refs, because they are not created yet.
The componentWillMount() is a chance for us to handle configuration, update our state, and in general prepare for the first render.
This means we can start performing calculations or processes based on the prop values.