code: function sumBy(array, iteratee) {
          return array && array.length ? baseSum(array, getIteratee(iteratee)) : 0;
        }
docs: 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).
  
  @static
  @memberOf _
  @category Math
  @param {Array} array The array to iterate over.
  @param {Function|Object|string} [iteratee=_.identity] The iteratee invoked per element.
  @returns {number} Returns the sum.
  @example
  
  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