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

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

Comments

[Sung M. Kim]

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...

[nigel447]

thanks Sung really helpful article

[Sung M. Kim]

You're welcome :)

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

[Frank Lemanschik]

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 !

[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"

[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.

[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...

[fidaay]

Dude, the functional component code is not well done. Try mutating the state just as you did in the class component, you'll see the exactly same behavior, do it as setState({ data: 0}), the value doesn't matter it will re-render.

[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 🤔

[smac89]

The example given there is not correctly done.

Here is an updated example which uses useEffect and also shows the last time the component was updated

stackblitz.com/edit/react-xeofau

[fidaay]

That's the same thing lol

[sunflowerseed]

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

[Kevin Sullivan]

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.

[Frank Lemanschik]

yes thats correct look for example dev.to/frankdspeed/the-html-compon...

[Mohamed Lamine Allal]

Unless the Class extends PureComponent

[John Miller]

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