Install 19, as your client requested, and use it like <=18. More on reddit.com

Unless there’s some specific API of React 19 you know you’re going to be heavily reliant on, I’d just stick with 18. The upgrade for the vast majority of apps will require essentially no effort. Keep in mind also that React 19 has been RC since April. I was tempted to upgrade back then thinking the full release would be coming “soon.” Here we are 6 months later and the final release state is still unknown. Don’t create a potential dependency headache for no reason. Just take note of any spots in your app you know you’d like to change when you upgrade to 19 eventually (for example, I flagged some usages of “useContext” that I’ll be able to move out of component render scope directly into event handlers). It was like 2 spots total on my most recent app. More on reddit.com

Go deeper. This stuff needs to all feel natural to you, for an interview. I run interviews often and people say they know this stuff, and can’t write or explain a use effect. Also, learn JavaScript, async await, and handling promises. Other promise functions too. Likt promise.all for example More on reddit.com

Lol, man if you want an "overwhelming" release, head over to Svelte, they are still fighting a civil war over the version 5 release. Or Vue, they still haven't gotten over the version 2 to version 3 transition. React is mature, stable, and boring. Those are the quality that gets the job done. More on reddit.com