Bump to v4.0.0.

assignInWith.js

@@ -0,0 +1,36 @@
+import copyObjectWith from './internal/copyObjectWith';
+import createAssigner from './internal/createAssigner';
+import keysIn from './keysIn';
+
+/**
+ * This method is like `_.assignIn` except that it accepts `customizer` which
+ * is invoked to produce the assigned values. If `customizer` returns `undefined`
+ * assignment is handled by the method instead. The `customizer` is invoked
+ * with five arguments: (objValue, srcValue, key, object, source).
+ *
+ * **Note:** This method mutates `object`.
+ *
+ * @static
+ * @memberOf _
+ * @alias extendWith
+ * @category Object
+ * @param {Object} object The destination object.
+ * @param {...Object} sources The source objects.
+ * @param {Function} [customizer] The function to customize assigned values.
+ * @returns {Object} Returns `object`.
+ * @example
+ *
+ * function customizer(objValue, srcValue) {
+ *   return _.isUndefined(objValue) ? srcValue : objValue;
+ * }
+ *
+ * var defaults = _.partialRight(_.assignInWith, customizer);
+ *
+ * defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
+ * // => { 'a': 1, 'b': 2 }
+ */
+var assignInWith = createAssigner(function(object, source, customizer) {
+  copyObjectWith(source, keysIn(source), object, customizer);
+});
+
+export default assignInWith;