_baseSum.js

define([], function() {

  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
  var undefined;

  /**
   * 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.
   */
  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;
  }

  return baseSum;
});