jQuery's not so great API for promises

jquery_promise.coffee

    post: (endpoint, params = {})->
      defer = $.Deferred()

      $.when(
        @requestChallenge()
      ).then( 
        (res) =>
          @authChallenge(res)
        ,
        (res) =>
          defer.reject(res)

      ).then(
        (res) =>
          @_post endpoint, (_.extend {}, res, params)
        ,
        (res) =>
          defer.reject(res)

      ).then(
        (res)->
          defer.resolve(res)
        ,
        (res)->
          defer.reject(res)
      )
    
      return defer.promise()