loglevel print duplicate log in React component about loglevel

[pimterry]

I can reproduce this, in codesandbox but also locally by exporting that same demo.

Testing it manually with a debugger, in fact it's the console.log that's wrong here - that App() method really is called twice.

It turns out that in React v17 a recent feature hides the output of console.log during a new second rendering pass. That hides the console.log output, but not loglevel's output, which holds a reference to console.log from beforehand.

More detail available in facebook/react#21783. This was fixed (moving the logic to React itself into React devtools, and showing the output but greyed out) in facebook/react#22030, but it sounds like that will only be included in React v18, not v17.

I'm going to close this - loglevel is working correctly, this is a React issue.

[gaearon]

React 18 is out, and double logs are no longer suppressed by default.
(But they're slightly dimmed if you have DevTools installed.)

See facebook/react#21783 (comment).

[pimterry]

I'm not aware of anything that could cause this. It's hard to debug this in more depth without a repro though. Can you share a playground or similar where I can test this behaviour myself?

[rsi-yhe]

https://codesandbox.io/s/empty-sun-5loi1?file=/src/App.js
I put two logs inside App.js and you can see console have 2 output from loglevel

[pimterry]

Great, thanks for the update @gaearon!