Hide certain log messages that pass through custom layout function

[mihaibrana]

Hello :). Thank you for porting & creating this library. Excellent work, that has helped me a great deal :).

I'm using my own layout for an appender.

https://github.com/log4js-node/log4js-node/blob/master/docs/layouts.md

I'd like to filter-out certain LogEvents based on the content (the log message itself), something that is not possible with typical filtering (which is usually limited at levels & categories).

Unfortunately, when I return "", I still get an empty line in the console (which makes sense, since log4js prints whatever the log function returns. But I'm wondering if there's a way to tell log4js to hide a message completely.

Ideally, either when I return "null" or empty string, I would like it to not display anything. Most likely "null" would be a better candidate for this.

I've taken a (short!) look in:

https://github.com/log4js-node/log4js-node/blob/master/lib/layouts.js

But can't figure out exactly where custom layouts are implemented.

[quocvu]

I would also be interested in knowing this. I'd like to filter based on user-agent. While debugging on production, the log entries from the crawlers are interfering.

[lamweili]

@quocvu I have replied to your user-agent filtering in #1214.