If we call the setState() function of a component in React, Browser re-render the whole DOM? or re-render only that component?
Updated
What I want to know is that whether Browser re-render(re-paint) the whole DOM(Page) when render() of only one component is called or Browser re-render only that Node.
That's not the case setState not only calls the render() function but after setState, the following lifecycle functions will run in order depending on what shouldComponentUpdate returns
if shouldComponentUpdate returns true(which is true by default).
shouldComponentUpdate (returns Boolean true/false)
componentWillUpdate (if true is returned)
render() (if true is returned)
componentDidUpdate (if true is returned)
It only triggers the re-render for the current component and all its children(considering no implementation of shouldComponentUpdate for any of its children), The process is called reconcilation.
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React components have a render method and can have a componentDidUpdate method. My question is: does componentDidUpdate get called after every render, and are there any times where componentDidUpdate gets called but render is not?
componentDidUpdate is always called after render. So if you prevent render with shouldComponentUpdate for example, didUpdate will never get called
Are there any times where componentDidUpdate gets called but render is not?
No, componentDidUpdate depends on render to be invoked. But the oposite can happen, componentDidUpdate doesn't get called after the first render
I am updating a legacy component which uses:
shouldComponentUpdate() to avoid an expensive state re-computation
componentWillUpdate() to do the re-computation and render if 1 passes
The docs say
if shouldComponentUpdate() returns false, then
UNSAFE_componentWillUpdate(), render(), and componentDidUpdate() will not be invoked`
But getDerivedStateFromProps() is not mentioned. I want to use this to replace the deprecated componentWillUpdate().
My question is: does shouldComponentUpdate() block the invocation of getDerivedStateFromProps() in the same way as it did for componentWillUpdate()?
NB: I know about looking at memoization as another solution.
When the passed props change getDerivedStateFromProps() function is called and then shouldComponentUpdate().
However if state changes only the shouldComponentUpdate() lifecycle function is triggered.
You can check the component lifecycle diagram here for more help
I was having some trouble figuring out where exactly the componentWillUpdate triggers. The docs say it triggers before render. However in my component the only props changing are defaultValue, which does not affect the DOM on udpate. So how come my componentWillUpdate is triggering? I was thinking it only triggered after props/state were tested to see if it actually changed the virtual DOM.
ComponentWillUpdate() has no bearing on whether the DOM will update. It's used only as a hook to perform some operations to your props before their rendered. If you want to prevent a component from rendering if the props change then you need to return false inside shouldComponentUpdate() note ComponentWillUpdate() is not called on first run (mounting) for that you want to use
componentWillMount()
In below image we have react life cycle methods.
Is there any other method which will call only when we change the state only ?
Here shouldComponentUpdate & comonentWillUpdate both will call in case of changes into props and state.
Thank you in Advance !!!
No lifecycle methods exist, but you can use the existing shouldComponentUpdate and componentWillUpdate hooks and wrap your state change logic in a check to make sure that previousState !== this.state
When you change your state, shouldComponentUpdate will be called, if this method return false. No other lifecyle methods will be called anymore.
So just check and return false in shouldComponentUpdate if you don't want other lifecycle methods to be called.
I have a component, and its componentDidUpdate seems to trigger for no reason. It's a child component, has no state. Even though the prevProps and this.props are the exact same its triggering, is there any reason for this? I thought update should only trigger when there is a change in the props/state?
componentDidUpdate() is fired every time the parent component re-renders (and passes in new props). And in stateful components also whenever setState() is fired.
Even if old prevprops and this.props are exactly the same, componentDidUpdate() is still fired if the parent component re-renders.
If you want to prevent react to enter a render cycle, you should implement the shouldComponentUpdate() method, where you can compare new and old props.
update should only trigger when there is a change in the props/state
No, React renders to VirtualDOM everything each time. But then it updates only changed properties in real DOM.
To avoid that you can use PureRenderMixin or your custom comparation code in shouldComponentUpdate.