Bump to v4.0.0.

sumBy.js

@@ -0,0 +1,35 @@
+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;
+});