I had some trouble isolating the specific line of code. It is internal
to NodeJS and part of the implementation of process.nextTick.
https://github.com/joyent/node/blob/master/src/node.js#L180-205
In Q, we call NodeJS’s process.nextTick to defer a callback into the
next turn of the event loop. We also nextTick implementations shimmed
for modern and legacy browsers, so the stack will vary from platform
to platform. In legacy browsers, it falls back to setTimeout, which
has a standardized minimum lag of some milliseconds. In newer
browsers, we use a hack based on a behavior of MessagePort that
side-steps the minimum delay.
Kris Kowal