code: function baseSum(array, iteratee) {
      var result,
          index = -1,
          length = array.length;
  
      while (++index < length) {
        var current = iteratee(array[index]);
        if (current !== undefined) {
          result = result === undefined ? current : (result + current);
        }
      }
      return result;
    }
docs: The base implementation of `_.sum` and `_.sumBy` without support for
  iteratee shorthands.
  
  @private
  @param {Array} array The array to iterate over.
  @param {Function} iteratee The function invoked per iteration.
  @returns {number} Returns the sum.