Ciro Santilli @cirosantilli 34

 Incoming links: Cannot update a component while rendering a different component warning in React

React  Updated 2024-12-15  +Created 1970-01-01

React officially recommends that you use Next.js ^[ref], so just do it. It just sets up obvious missing functionality from raw React.

React feels like a good. But it also feels impossible to use/learn sometimes.

Its main design goal is to reduce DOM changes to improve rendering times.

And an important side effect of that is that it becomes easier to do stuff of the type:

user creates a new comment that appears on screen without page reload
comment has a delete button, which is JavaScript callback activated

and then the new comment easily gets the callback attached to it.

And it also ends up naturally doubling as a template engine.

But React can also be extremely hard to use. It can be very hard to know what you can and cannot do sometimes, then you have to stop and try to understand how react works things better:

cannot update a component while rendering a different component warning in React
Rendered more hooks than during the previous render.
cannot use hooks from helpers:

The biggest problem is that it is hard to automatically detect such errors, but perhaps this is the same for other frontend stuff. Though when doing server-side rendering, the setup should really tell you about such errors, so you don't just discover them in production later on.

Is is also very difficult to understand precisely why hooks run a certain number of times.

Examples under: react.

react/hello.html
react/hello-func.html: Hello World with a React function component instead of classes. At page load console shows:
```
Main
```
and then after each click:
```
onClick
Main
```
so we understand that Main insanely functions both as the constructor and as the render function in React function components.
react/hello-func-use-callback.html: same as react/hello-func.html but with useCallback. TODO no advantages in this case? When does it help?
react/hello-without-jsx.html: Hello World in pure JavaScript, without JSX. Exactly equivalent to react/hello.html. Documented at: reactjs.org/docs/react-without-jsx.html Understanding this is fundamental to understanding React.
react/prop-change.html: shows what gets called as parameters flow down through the tree.
By looking at the console, we see all render get called every time, even if props didn't change, but not the constructors.
After page load the console contains:
```
Main.constructor
Main.render
NotMain.constructor
NotMain.render
NotMain2.constructor
NotMain2.render
```
Then, every time we click the button it adds:
```
handleClick
Main.render
NotMain.render
NotMain2.render
```
Note how the props of NotMain only change every other click, but render still gets called every time.
In order to make React not re-render when there are not changes, you have to either:
- define the shouldComponentUpdate method of class components
- wrap functional components in React.memo
Related:
- stackoverflow.com/questions/61301937/why-is-react-component-rerendering-when-props-has-not-changed
- stackoverflow.com/questions/67214500/why-react-rerendres-components-even-if-props-didnt-changed
react/prop-change-hook.html: same as react/prop-change.html, but using hooks. The notable difference is that functional components don't have a clear constructor/render separation, the function just gets called every time. Then React does some magic to ensure that useState returns the current state, except for the first render where they return the initial value.
react/prop-change-hook-use-memo.html: TODO forgot if this example is useful, was tring to use useMemo
react/prop-change-child.html: shows what child prop changes do not call render on parent, Main does not show up on console when you click under NotMain
react/hook-from-function-fail.html: TODO got some errors that seemed linked to this on a larger program, but failed to minimize them here
react/hook-different-number-of-times.html: this illustrates one of the cardinal points of using hooks: you must always call them the same number of times, otherwise it fails with:
React has detected a change in the order of Hooks called by Main. This will lead to bugs and errors if not fixed.
In the case of useState, we can kind of understand why this happens: React must use the order of calls to determine which state variable to return at each point in time.
react/hello-hook-use-effect.html: just checking when it gets called. Happens after every render
```
handleClick
Main
useEffect
useEffect2
```
TODO create a test \a[react/img-broken.html]
- stackoverflow.com/questions/34097560/react-js-replace-img-src-onerror
- stackoverflow.com/questions/36305805/how-to-hide-alt-text-using-css-when-the-image-is-not-present

How React works bibliography:

www.netlify.com/blog/2019/03/11/deep-dive-how-do-react-hooks-really-work/ shows how uesState works under the hood with crazy closures
medium.com/@gethylgeorge/how-virtual-dom-and-diffing-works-in-react-6fc805f9f84e

Video 1.

React for the Haters in 100 Seconds by Fireship (2022)

Source.

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