Thank you for the feedback. I agree this behavior was confusing. (We changed it to slightly dimming the second log in response to the community discussion. This change is released in React 18.) The new behavior seems like the tradeoff we dislike the least but new ideas are always welcome.
Overall, we’ve found that calling some methods twice is a crude but effective way to surface a class of bugs early. I absolutely agree that it’s not the best possible way to do that, and we’re always thinking about ways to surface that earlier — for example, at the compile time. But for now, the current solution is the best we have found in practice.
Thanks for clarifying Dan. I think it might be appropriate for React to console.warn that strict mode is active (in dev mode) with a short description of this side effect, or perhaps the “dimmed” log message could have an explanation after it. Not all users have explicitly enabled strict mode (e.g. in my case I just started a new Next.js project) so this behaviour can be quite surprising and hard to track down why it’s happening.
The problem with logging a message is that it creates permanent console noise for people who enable it, thereby incentivising people not to. It seems like a small thing but people are very sensitive to extra logs. And if you allow silencing the log, we’re back to where we were.
I guess I was imagining a single warning message at the top, “React is running in strict mode. This may result in you seeing console log messages twice. Read more here. Click here to disable this message for this site. Click here to disable this message globally”. Not too much noise really, especially if you can disable it per site or globally.
I do see where you are coming from, I’m just thinking of my experience where I spent 30 mins+ thinking I’d found a bug or fundamentally misunderstood React all these years or whatever, and it turned out it was just the strict mode - but because I was using Next.js I never explicitly opted in, so had no way of knowing this might start happening (unless I read every release note). I’m guessing a lot of other developers using Next might be similarly confused!
Overall, we’ve found that calling some methods twice is a crude but effective way to surface a class of bugs early. I absolutely agree that it’s not the best possible way to do that, and we’re always thinking about ways to surface that earlier — for example, at the compile time. But for now, the current solution is the best we have found in practice.