Custom JavaScript function queue
I'm still trying to wrap my head around what exactly is happening in that apply/call mass :-)

But that may be a hint, that those three lines aren't really well readable. I'm aware you want to keep it small, but unless the code needs to be really, really fast (which I doubt it does) I would prefer readability over shortness.

In that line, I would prefer to have long, more expressive variable names. It be better to use complete words instead of Q, t, n, a etc.

If in the end, shortness is still important, then use a JavaScript compressor.

BTW, you should keep JavaScript naming conventions in mind, too. Constructor functions ("classes") should be named with capital letters (Q instead of q) and fields should use small letters (q instead of Q).

Just out of interest: Can you give a concrete use case for this kind of queue?