Angular directive for tracking scroll events

ng-scroll-support.coffee

angular.module 'ngScrollSupport', []
.directive 'ngScrollSupport', ->
  restrict: 'A'
  link: (scope, element, attrs) ->
    atTop = true
    atBottom = false
    check = ->
      scrollTop = element.prop 'scrollTop'
      scrollHeight = element.prop 'scrollHeight'
      clientHeight = element.prop 'clientHeight'
      switch
        when scrollTop is 0 and not atTop
          atTop = true
          element.addClass 'at-top'
        when scrollTop is scrollHeight - clientHeight and not atBottom
          atBottom = true
          element.addClass 'at-bottom'
        else
          atTop = false
          atBottom = false
          element.removeClass 'at-top'
          element.removeClass 'at-bottom'
    element.addClass 'at-top'
    element.on 'scroll', _.throttle check, 300
    scope.$on '$destroy', ->
      element.off 'scroll'

Was just playing around with it, but I don't want to loose it.