What’s new in Create-React-App v4 & React 17

Patrick Ullrich
4 min readOct 30, 2020


React reached a new major version on October 20th (https://reactjs.org/blog/2020/10/20/react-v17.html) and 4 days later one of our favorite React scaffolding tools Create-React-App followed suit with the release of version 4, that includes React 17.

I dug through the features highlighted in the respective blogs and followed PRs to summarize all features that I am most excited for.

New JSX Transformation

The React team worked with the Babel team to bring a neat new update to the react/jsx-runtime and react/jsx-dev-runtime in Babel version 7.9.0, which allows us to omit import React from 'react'; from the top of our .jsx / .tsx files. Your index file will be the only file that still needs the import. You can read more about the new jsx transformation here.

Event Pooling

I didn’t find this change under the highlights, but I think it’s an important one to point out. React event handlers are passed into instances of SyntheticEvent , these are then pooled in the Event Pool.

(If you want to know more about why this was done, check out this Stackoverflow answer. https://stackoverflow.com/a/53500357/1467478)

It was the cause of many headscratchers. Imagine following simplified debounce functionality:

const [search, setSearch] = useState('');const onChange = (e) => {
setTimeout(() => setSearch(e.target.value), 250)} // :(
return (
<input type="text" onChange={onChange} />
<p>Search Text: {search}</p>

In React 16 and older, this code would throw an error of Cannot read property 'value' of null , on setTimeout(() => setSearch(e.target.value), 250)}, but it does work as expected in React 17.

Web Vitals

If you have measured performance in your react app for Google Analytics, you may have used web-vitals before.

“The web-vitals library is a tiny (~1K), modular library for measuring all the Web Vitals metrics on real users, in a way that accurately matches how they're measured by Chrome and reported to other Google tools (e.g. Chrome User Experience Report, Page Speed Insights, Search Console's Speed Report).” — https://github.com/GoogleChrome/web-vitals

It allows us to measure metrics for:

Create-React-App’s (CRA) boilerplate now comes set up to record those measurements out of the box. If you want to see the values right now, checkout your index.js and pass console.log into the reportWebVitals function.

Displaying web-vitals output in console

Furthermore, CRA now has a measuring-performance documentation page giving examples on how to utilize the new reportWebVitals function to log to Google Analytics or your own backend.


My personal favorite update coming with v4 is react-refresh enabled by default. If you haven’t heard of react-refresh, it essentially will give you the experience of hot-reload , but your application will preserve its state.

But it doesn’t just stop there, it also only re-renders the required React components, without reloading the entire app and reloads once you resolved a syntax or run time error without needing you to manually refresh.

Ever worked on a workflow and you had to style some pieces on the very last step? On every change you have to click through the entire flow again, just to see if your change has done what you wanted.

Sure… You might render the component somewhere else in the app or even better, you worked on the component in isolation and this isn’t even a problem. But sometimes, you didn’t. And you start coming up with quick workarounds, such as stringifying the state, logging it, and intializing the component with it… so that on every page reload you are right back where you want it?

…No? Nobody? … Just me?

Anyhow, react-refresh swoops in and saves the day.

You can find more information on this feature here in the PR by @charrondev https://github.com/facebook/create-react-app/pull/8582.

https://github.com/facebook/create-react-app/pull/8582 by https://github.com/charrondev

You will see in the PR the mention of adding FAST_REFRESH=TRUE

TypeScript 4 Support

If you’re using npx create-react-app --template=typescript you are probably excited to hear that CRA 4 is coming with TypeScript 4 support. I don’t think I could ever do TypeScripts announcement blog posts justice, but some noteworthy features are:

  • More Tuples features such as Labeled Tuple Elements
  • Short Circuiting Assignment Operators
  • Custom JSX Factories
  • Handy Optional Chaining Refactoring

That being said, it seems as of writing of this article there are currently some errors with the TypeScript template. I actually encountered the same issue today while upgrading one of our projects.

Cannot add property noFallthroughCasesInSwitch, object is not extensible on verifyTypeScriptSetup.js:210. There are some Workarounds suggested that you can give a try, otherwise hopefully this will be resolved soon.

Follow here to get an update on this issue: https://github.com/facebook/create-react-app/issues/9429

ESLint v7.0.0 & Jest 26

Both ESLint and Jest have gotten an upgrade. Personally I didn’t find anything that really stood out to me, but definitely check out the release notes here:

I can’t wait to update some bigger projects to includereact-refresh and reportWebVitals will definitely replace my hacked together solution for Google Analytics from a year ago. I am also weirdly excited to omit import React from 'react' on top of my .jsx files going forward.

What is your favorite new feature in React 17 and CRA 4? Did I miss any highlights? Let me know in the comments.



Patrick Ullrich

he/him • ReactJS • .NET Core • Software Developer • Rowing • Writing about Software & Life