From the fine manual:
As of jQuery 1.4.3, an optional string naming an easing function may be used.
So the easing
argument should be a string which names the easing to use, not an easing function itself. Furthermore, this is a method call:
@easing()
while this is a reference to your easing function:
@easing
If you want to define a custom easing, you have to do it globally by adding a property to $.easing
:
$.easing.whatever = (x, t, b, c, d) ->
#...
and then you reference it by name:
@container.slideDown @config.animationTime, 'whatever', () => ...