CoffeScript - sumar propiedades de un JSON

sum_attributes = (x) => 
sum = 0
for name, value of object.properties
sum += value.attribute_values[x]
sum
alert sum_attributes('a')
alert sum_attributes('b')

sum = (arr) -> _.reduce arr, ((memo, num) -> memo + num), 0 sum _.pluck(object.properties, 'a')