Where can I find documentation for Ember nearestOfType() function

I've searched through Google, etc. and can not find more than a cursory reference. What does this function do?

I am trying to figure out how the ember-bootstrap add-on works, and came upon this function here: http://ift.tt/2hsGxGl

I think I know what the code is doing, I've just never seen this nearestOfType() function before.