Bump to v4.0.0.

internal/baseSum.js

@@ -1,19 +1,22 @@
-import baseEach from './baseEach';
-
 /**
- * The base implementation of `_.sum` without support for callback shorthands
- * and `this` binding.
+ * The base implementation of `_.sum` without support for iteratee shorthands.
  *
  * @private
- * @param {Array|Object|string} collection The collection to iterate over.
+ * @param {Array} array The array to iterate over.
  * @param {Function} iteratee The function invoked per iteration.
  * @returns {number} Returns the sum.
  */
-function baseSum(collection, iteratee) {
-  var result = 0;
-  baseEach(collection, function(value, index, collection) {
-    result += +iteratee(value, index, collection) || 0;
-  });
+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;
 }