What's new in React 18 ?
Tags: react, reactnative, javascript, react18
Published: 2021-08-23 11:15:32

While there is still a lot more to come, I thought may be fascinating to share its top new features.

### Automatic batching

Batching in React is whenever multiple state updates are combined into a single re-render.

```jsx
function App() {
  const [count, setCount] = useState(0);
  const [flag, setFlag] = useState(false);

  function handleClick() {
    setCount(c => c + 1); // Does not re-render yet
    setFlag(f => !f); // Does not re-render yet

    // React will only re-render once at the end (that's batching!)

  }

  return (
    <div>
      <button onClick={handleClick}>Next</button>
      <h1 style={{ color: flag ? "blue" : "black" }}>{count}</h1>
    </div>
  );
}
```

In this example, the component would only be rendered once after **handleClick** is called, Although we might think **setCount** and **setClicked** would trigger two re-renders.

Now, while this works well at the moment, 

if you’re calling multiple state updates in a different context such as **a promise** or **a callback.**

```jsx
// Promise
fetchSomething().then(()=> {
	setCount(count+1);
	setFlag(true);
})

//callback or timeout
setTimeOut(()=>{
	setCount(count + 1);
	setFlag(true);
})
```

**React won’t batch these two updates into one and you’ll get two re-renders when only one would have been needed.**

With React 18, all these use-cases will now be covered and state updates will be batched automatically no matter what’s the context.

```jsx
import {unstable_batchedUpdates} from 'react-dom';
unstable_batchedUpdates(() => {
	setCount(count + 1);
	setFlag(true);
})

//React 18 will do it for you by default. 
```

This might remind you of **unstable_batchedUpdates** that was doing exactly this, well now, React will do it for you by default.

**How to stop batching ?**

If you happen not to want these updates to be batched, you’ll need to use **flushSync** that will re-render your component every time it’s done running the function you’re passing it to.

```jsx
import {flushSync} from 'react-dom';

function handleClick(){
	flushSync(()=> {
			setCount(count + 1);
	});
	// React has re-render

	flushSync(()=> {
			setFlag(true);
	});
	// React will re-render
}
```

So with the following, your component would render twice instead of just one time.

### Transitions

A really big new feature, which allows you “to tell React **which updates are urgent, and which are not**”.

A good example of that is a search input that should filter out a list of elements.

So if you’re updating a search input, you’d want its value to change as we type, Although the search results might appear in a second

phase when we’re done typing.

```jsx
import {startTransition} from 'react';

//Urgent : Update input value as type
setInputValue(input);

startTransition(()=>{
	//Secondary: Show the search results
	setSearchQuery(input)
});
```

This is where we could mark the input value change as an urgent update, and the elements filtering as secondary also called now a transition.

Transitions can be interrupted by urgent updates and previous transitions that are no longer relevant will be dismissed.

This allows the user interface to only show its most up-to-date state and skip secondary updates, transitions, that might be slower

to compute and sometimes return intermediate states that are irrelevant.

As you can see here, we’re marking the input value change as urgent, and run our secondary update inside a transition as it might trigger slow computations and could freeze or slow down the whole user experience as we type.

**startTransition** is great for any update “you want to move to the background” such as slow and complex rendering tasks or when

updates rely on fetching data that might take time due to a slow network.

### Suspense and Server-Side Rendering

The way SSR ( Server side rendering ) works is by rendering all the

components on the server first, then sending the result as HTML to the browser.

After that, the JavaScript is loaded as usual and the HTML magically becomes interactive by what is called hydration.

This turns your static HTML elements into your dynamic React components as you know them.

**Problem?**

The main problem with this approach is that as long as the JavaScript hasn’t been fetched, loaded, and your HTML hydrated, your page won’t be interactive.

To solve this waterfall issue, React 18 now offers two new features for SSR: 

**Streaming HTML and Selective Hydration**

### Streaming HTML

Streaming HTML means that the server can send pieces of your components as they get rendered.

This works by using Suspense, where you’d say which parts of your application will take longer to load and which ones should be rendered directly.

```jsx
<Page>
	<Article />
	<Suspense fallback={<Loader />}>
		 <Comments />	
	</Suspense>
</Page>
```

If you think of an article with comments where the article is the critical part of the page, you could say load the article but don’t

wait for the comments to be ready to send HTML to the browser.

You could show a spinner instead using **Suspense** and once the comments are ready,

React will send new bits of HTML that will replace the spinner in place.

### Selective hydration

Selective hydration is awesome.

Where before you’d have to wait for every component to be rendered to begin hydration, but now components wrapped with Suspense won’t block hydration anymore.

if we go back to our article page, the comments that we wrapped with Suspense won’t block the article and other components to be hydrated.

Every ready component will start hydrating and the comments will too, once the browser gets both its content and JavaScript code.

The most important feature of selective hydration

is that if you happen to interact with one of the components before it’s been fully hydrated, meaning you click somewhere, for example, React will prioritize this component’s hydration.

This ensures that the most critical interactions are to be repeated as soon as we can hydrate the related component, making sure it’s

hydrated before the others.

Thank you for reading. 

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