lodash/sumBy.js

define(['./internal/baseIteratee', './internal/baseSum'], function(baseIteratee, baseSum) {

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

  /**
   * 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
   *
   * // using the `_.property` iteratee shorthand
   * _.sumBy(objects, 'n');
   * // => 20
   */
  function sumBy(array, iteratee) {
    return (array && array.length)
      ? baseSum(array, baseIteratee(iteratee))
      : undefined;
  }

  return sumBy;
});