             return i if equalsFunc y, x
         -1
 
-    lazyMap: (f) ->
+    lazyMap: (f = _.identity) ->
         # Reactive
         if Tracker.active
             J.AutoList(

                        @get i
                    else
                        lst.get i - @size()
            )
        else
            J.List @getValues().concat lst.getValues()


    filter: (f = _.identity) ->
        # Reactive
        J.List _.filter @map().getValues(), f

    forEach: (f) ->
        # Reactive
