React

Website: reactjs.org

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 extremelly 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.

React JSX (JSX)

React error

words: 2 articles: 1

Cannot update a component while rendering a different component warning in React

words: 2

Minimal reproduction: stackoverflow.com/questions/62336340/cannot-update-a-component-while-rendering-a-different-component-warning/70317831#70317831

React example

words: 93 articles: 1

react/hello.html

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Minimal React hello world example. As you click:

one counter increments every time
the other increments every two clicks

By opening a web inspector, you can see that only modified elements get updated. So we understand that JSX parses its "HTML-like" into a tree, and then propagates updates on that tree.

By looking at the terminal, we see that render() does get called every time the button is clicked, so the tree of elements does get recreated every time. But then React diffes thing out and only updates things in the DOM where needed.

react/hello.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<script src="https://unpkg.com/react@17/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone@7.14.7/babel.min.js"></script>
</head>
<body>
<p><a href="https://cirosantilli.com/_file/react/hello.html">https://cirosantilli.com/_file/react/hello.html</a></p>
<div id="root"></div>
<script type="text/babel">
class Main extends React.Component {
  constructor(props) {
    super(props)
    console.log('Main.constructor')
    this.state = {n: 0}
  }

  render() {
    console.log('Main.render')
    return (
      <div>
        <button
          onClick={
            () => {
              console.log('onClick')
              this.setState((state, props) => ({
                n: state.n + 1
              }))
            }
          }>Click me</button>
        <div>n = {this.state.n}</div>
        <div>n/2 = {Math.floor(this.state.n / 2)}</div>
      </div>
    )
  }
}
ReactDOM.render(<Main/>, document.getElementById('root'))
</script>
</body>
</html>

React DOM mainpulation

words: 108 articles: 2

react/ref-click-counter.html

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Dummy example of using a React ref This example is useless and to the end user seems functionally equivalent to react/hello.html.

It does however serve as a good example of what react does that is useful: it provides a "clear" separation between state and render code (which becomes once again much less clear in React function components.

Notably, this example is insane because at:

<button onClick={() => {
  elem.innerHTML = (parseInt(elem.innerHTML) + 1).toString()

we are extracing state from some random HTML string rather than having a clean JavaScript variable containing that value.

In this case we managed to get away with it, but this is in general not easy/possible.

react/ref-click-counter.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>ref click counter</title>
<script src="https://unpkg.com/react@17/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone@7.14.7/babel.min.js"></script>
</head>
<body>
<div id="root"></div>
<script type="text/babel">
class Main extends React.Component {
  constructor(props) {
    super(props)
    console.log('Main.constructor')
    this.counterElem = null
  }

  render() {
    console.log('render')
    return (
      <div>
        <button onClick={() => {
          console.log('onClick')
          elem = this.counterElem
          elem.innerHTML = (parseInt(elem.innerHTML) + 1).toString()
        }}>
          Click me
        </button>
        <div
          ref={
            (elem) => {
              console.log(`ref callback ${elem}`)
              this.counterElem = elem
            }
          }
        >
          0
        </div>
      </div>
    )
  }
}

ReactDOM.render(
  <Main/>,
  document.getElementById('root')
)
</script>
</body>
</html>

react/ref-click-counter-func.html

words: 4

React function component version of react/ref-click-counter.html using useRef.

react/ref-click-counter-func.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>ref click counter</title>
<script src="https://unpkg.com/react@17/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone@7.14.7/babel.min.js"></script>
</head>
<body>
<div id="root"></div>
<script type="text/babel">
function Main() {
  console.log('Main')
  const counterElem = React.useRef(null)
  return (
    <div>
      <button onClick={() => {
        console.log('onClick')
        const elem = counterElem.current
        elem.innerHTML = (parseInt(elem.innerHTML) + 1).toString()
      }}>
        Click me
      </button>
      <div
        ref={
          (elem) => {
            counterElem.current = elem
          }
        }
      >
        0
      </div>
    </div>
  )
}
ReactDOM.render( <Main/>, document.getElementById('root'))
</script>
</body>
</html>

React class vs function component

words: 59 articles: 6

React function components do produce shorter code. But they are also impossible to understand without knowing what is their corresponding class component.

Hooks were introduced much after classes, and just require less code, so everyone is using them now instead of classes.

React class component

React function component

words: 20 articles: 4

React hook

words: 20 articles: 3

`useEffect`

words: 20

This should only be used for things that happen outside of the state that React trackes, e.g. window event handlers.

Examples:

react/ref-click-counter-func.html

`useRef`

`useCallback`

Next.js

words: 705 articles: 3

Framework built on top of React.

Officially recommended by React ^[ref]:

Recommended Toolchains
If you’re building a server-rendered website with Node.js, try Next.js.

gothinkster/realworld blog example by Ciro Santilli: node Express Sequelize Next.js realworld example app.

Basically what this does is to get server-side rendering just working by React, including hydration, which is a good thing.

Next.js sends the first pre-rendered HTML page along with the JavaScript code. Then, JavaScript page switches just load the API data.

Next.js does this nicely by forcing you to provide page data in a serialized JSON format, even when rendering server-side (e.g. the return value of getServerSideProps). This way, it is also able to provide either the full HTML, or just the JSON.

Some general downsides:

it does feel like they don't document deployment very well however, especially non-Vercel options, which is the company behind Next.js. I'm unable to find how to use a non Vercel CDN with ISR supposing that is possible.
Next.js is very opinionated, and like any opinionated library it is sometimes hard to know why something is/isn't happening, and sometimes it is hard/impossible to do what you want with it unless they add support. They have done good progress, but even as of 2022, some aspects just feel so immature, some major-looking use cases are not very well done.

In theory, Next.js could be the "ultimate frontend framework". It does have a lot of development difficulties that need to be ironed out, but the general concepts, and things it tries to integrate, including e.g. webpack, TypeScript, etc. are good. Maybe the question is when will someone put it together with an amazing backend library and dominate and finally put an end to the infinite number of Js Frameworks!

In-tree examples at: github.com/vercel/next.js/tree/canary/examples

canary is their master: github.com/vercel/next.js/issues/3211

In order to offer its amazing features, Next.js is also extremely opinionated, which means that if something wasn't designed to be possible, it basically isn't.

No prerender with custom server? It forces you to write your API with next as well? Or does it mean something else?

TODO can it statically generate pages that are created at runtime? E.g. if I create a new blog post, will it automatically upload a static page? It seems that yes, and that this is exactly what Incremental Static Regeneration means:

However, Ciro can't find any mention of how to specify where the pages are uploaded to... this is pat of the non-Vercel deployment problem.

Can't ISR prerenter by URL query parameters:

That plus the requirement to have one page per file under pages/ leads to a lot of useless duplication, because then you are forced to place the URL parameters on the pathnames.

"Module not found: Can't resolve 'fs'" Hell. The main reason this happens seems to be the that in a higher order component, webpack can't determine if callbacks use the require or not to remove it from frontend code. Fully investigated and solved at:

stackoverflow.com/questions/64926174/module-not-found-cant-resolve-fs-in-next-js-application/70363153#70363153

Overviews:

www.reddit.com/r/reactjs/comments/8evy5d/what_are_the_downsides_to_nextjs/ 2017 What are the downsides to Next.js?

TODO answer:

stackoverflow.com/questions/59674496/how-to-pass-pageprops-in-next-js-from-a-child-component-to-app-js

Next.js example

words: 217 articles: 1

Our oxamples are located under nodejs/next:

nodejs/next/hello-world: a hello world. There's an in-tree one at: github.com/vercel/next.js/tree/e75361fd03872b097e817634c049b3185f24cf56/examples/hello-world, but ours is truly minimal
nodejs/next/hoc: shows how to use a higher order component (HOC) to factor out getStaticProps across two pages: nodejs/next/hoc/pages/index.js and nodejs/next/hoc/pages/notindex.js
nodejs/next/typescript: simple TypeScript example, minimized from: github.com/vercel/next.js/tree/d61b0761efae09bd9cb1201ff134ed8950d9deca/examples/with-typescript
Notably, that shows how require errors are avoided in that case as mentioned at: stackoverflow.com/questions/64926174/module-not-found-cant-resolve-fs-in-next-js-application/70363153#70363153
nodejs/next/localStorage: a counter that is persistent across page reloads by using localStorage. Used in: stackoverflow.com/questions/54819721/next-js-access-localstorage-before-rendering-page/68136224#68136224

Solved ones:

solved by preview mode in Next.js 12:
- ISR was basically unusable for CRUD websites because you can't force a one-off immediate page update:
  - github.com/vercel/next.js/discussions/25677

nodejs/next/posts

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The goal of this example is to understand when states and effects happen when changing between different routes that use the same component.

Behaviour is follows:

visit: localhost:3001/1
click count++. This makes count: 1
click "2" to vist localhost:3001/2
outcome: count is still 1

This is likely because in React the state kept in the virtual DOM structure, and identical structure implies identical state. So when we change from post 1 to 2, we still have a Post object, and state is unchaged.

Next if we click:

"Index" to go to localhost:3000
"1" to go to localhost:3001/1

then the count is back to 0. This is because we changed the Post object in the DOM to Index and back, which resets everything.

Bibliography:

stackoverflow.com/questions/63143334/how-to-not-persist-state-between-next-js-dynamic-routes