Bump to v4.0.0.

sumBy.js

@@ -0,0 +1,32 @@
+import baseIteratee from './internal/baseIteratee';
+import baseSum from './internal/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 _
+ * @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;
+}
+
+export default sumBy;