I've opened an issue in github official site, but I have no response
You probably won't get any response.
The last check-in to that project was 3 years ago, there are 250 open Issues, and the author doesn't seem to respond to Issues - just helpful users. There's a blog with last entry in 2013. Users are begging for an update to work with Bootstrap 4, but clearly not coming.
I think it should be considered "abandonware". If you are lucky, somebody else will take over the project. There are quite a number of forks where others have made their own changes. You can explore the Network Graph here:
Maybe one will emerge as a maintainer going forward.
If it works for you right now, it works. But you should not expect it to work forever, as browsers and platforms change. At some point Bootstrap 3 will be a distant memory.
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Back to the problem, as Jake points out, you are supposed to return the response. It's intended for use in implementing a "filter" - that is, you might make changes to the response. But there's no reason it can't also be used as a "tap" as you intend.
Keep in mind that each response will overwrite your variable! Hopefully, you are done with it before the next one comes.
If you need to "do something" with the response, it would be better if you call a function to do whatever you need to do with it, rather than stuffing a variable that might quickly be overwritten by the next response.
On Firefox, do you get NO console.log output? Not even on the FIRST response?
(I'm not familiar with the plugin, not sure how frequent Ajax requests might be, or what triggers them.)
Start by fixing the return.