sumBy.js

var baseIteratee = require('./_baseIteratee'),
    baseSum = require('./_baseSum');

/**
 * 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 _
 * @since 4.0.0
 * @category Math
 * @param {Array} array The array to iterate over.
 * @param {Function} [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
 */
function sumBy(array, iteratee) {
  return (array && array.length)
    ? baseSum(array, baseIteratee(iteratee, 2))
    : 0;
}

module.exports = sumBy;