Centre align and element relative to parent (or window)

centreElement.js.coffee

$.fn.centre = (dims = ['left', 'top'], parent = window, offset = 0) ->
  that = @

  _.each dims, (dim) ->
    
    prop = 'width'  if dim == 'left'
    prop = 'height' if dim == 'top'

    parent_prop = eval("$(parent).#{prop}()")
    this_prop   = eval("$(that).#{prop}()")

    $(that).css("margin-#{dim}", (parent_prop - this_prop)  / 2) if dim and parent_prop and this_prop