This method is like _.sum except that it accepts iteratee which is invoked
for each element in array to generate the value to be summed. The iteratee
is invoked with one argument: (value).
var objects = [{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }];
_.sumBy(objects, function(o) { return o.n; });
// => 20
// The `_.property` iteratee shorthand.
_.sumBy(objects, 'n');
// => 20