Understanding Bootstrap 3 Collapse

Understanding Bootstrap 3 Collapse

i am trying to understand the source of bootstrap collapse plugin. Sorry
if i am asking a stupid question.
Collapse.prototype.dimension = function () {
var hasWidth = this.$element.hasClass('width');
return hasWidth ? 'width' : 'height'
}
what is this used for? why is it checking for width class? and i couldn't
find any method that add width or height class. it seems like it is always
returning 'height', then why not just return height directly?
thanks in advance.
-Roy