Bump to v4.0.0.

unionWith.js

@@ -0,0 +1,34 @@
+import baseFlatten from './internal/baseFlatten';
+import baseUniq from './internal/baseUniq';
+import isArrayLikeObject from './isArrayLikeObject';
+import last from './last';
+import rest from './rest';
+
+/**
+ * This method is like `_.union` except that it accepts `comparator` which
+ * is invoked to compare elements of `arrays`. The comparator is invoked
+ * with two arguments: (arrVal, othVal).
+ *
+ * @static
+ * @memberOf _
+ * @category Array
+ * @param {...Array} [arrays] The arrays to inspect.
+ * @param {Function} [comparator] The comparator invoked per element.
+ * @returns {Array} Returns the new array of combined values.
+ * @example
+ *
+ * var objects = [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }];
+ * var others = [{ 'x': 1, 'y': 1 }, { 'x': 1, 'y': 2 }];
+ *
+ * _.unionWith(objects, others, _.isEqual);
+ * // => [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }, { 'x': 1, 'y': 1 }]
+ */
+var unionWith = rest(function(arrays) {
+  var comparator = last(arrays);
+  if (isArrayLikeObject(comparator)) {
+    comparator = undefined;
+  }
+  return baseUniq(baseFlatten(arrays, false, true), undefined, comparator);
+});
+
+export default unionWith;