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ReactJs setState ReactJS -- If it is setting a state with the same value, will the component be re-rendered?

sunflower profile image sunflowerseed ・1 min read

Function component -- setValue(v)

if v is the same value as before, then the function component won't be re-rendered (the function component won't be called).

Example: https://stackblitz.com/edit/react-zo3hnj?file=index.js

Class component -- setState({ data: v })

if v is the same value as before, then the class component will be re-rendered (the class component's render() will be called).

Example: https://stackblitz.com/edit/react-fb8npx?file=index.js

Discussion (14)

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dance2die profile image
Sung M. Kim • Edited

Nice demos there~

It's a confusing concept well-demonstrated.

For Class component version this.setState({ count: value }); you are changing the "reference". For the Function Component, you are changing the primitive, thus the component doesn't re-render.

React re-renders as states change. States change when a "reference" change. Primitive values are stored in stack. This leads understanding of JS.

For more info on how FC & CC differ check out

overreacted.io/how-are-function-co...

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nigel447 profile image
nigel447

thanks Sung really helpful article

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dance2die profile image
Sung M. Kim • Edited

You're welcome :)

Dan's article's are long but well written and thorough.

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frankdspeed profile image
Frank Lemanschik • Edited

This is a amazing example why react sucks you never know what it does when ok sure if your the inventor of react or study its code you know it but who want's to follow that?

Good Example !

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anuraghazra profile image
Anurag Hazra

"This is a amazing example why react sucks" this statement is wrong!

It should be "This is an amazing example how javascript handles Objects" Or "This is an amazing example how weird js is"

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frankdspeed profile image
Frank Lemanschik

nope your wrong its react doing this let me explain the function component is a render function thats why it can not get called again and will not get called again the class component has a render function that can and will get called again.

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johnrobmiller profile image
John Miller

Unlike the semicolon in js, the period is NOT optional in the English language, and I don't the the devs are going to change this anytime soon...

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sunflower profile image
sunflowerseed Author

can you change the class component so that it behaves like useState(0)?

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wolverineks profile image
Kevin Sullivan • Edited

If you extend the class component from React.PureComponent it will do a shallow comparison on props and state to determine if it should rerender.

That being said, doing this is usually slower than allowing the component to rerender.

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andreabarghigiani profile image
Andrea Barghigiani

It's just me or it re-render the function component with Hooks as well? Tryed on both to set the new value to an integer (20) and still re-render both of them 🤔

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frankdspeed profile image
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johnrobmiller profile image
John Miller • Edited

This is strange design from the React developers. Knowing that one should remove confusing inconsistencies like this is very basic design practice.