Bump to v4.0.0.

unionBy.js

@@ -0,0 +1,36 @@
+import baseFlatten from './internal/baseFlatten';
+import baseIteratee from './internal/baseIteratee';
+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 `iteratee` which is
+ * invoked for each element of each `arrays` to generate the criterion by which
+ * uniqueness is computed. The iteratee is invoked with one argument: (value).
+ *
+ * @static
+ * @memberOf _
+ * @category Array
+ * @param {...Array} [arrays] The arrays to inspect.
+ * @param {Function|Object|string} [iteratee=_.identity] The iteratee invoked per element.
+ * @returns {Array} Returns the new array of combined values.
+ * @example
+ *
+ * _.unionBy([2.1, 1.2], [4.3, 2.4], Math.floor);
+ * // => [2.1, 1.2, 4.3]
+ *
+ * // using the `_.property` iteratee shorthand
+ * _.unionBy([{ 'x': 1 }], [{ 'x': 2 }, { 'x': 1 }], 'x');
+ * // => [{ 'x': 1 }, { 'x': 2 }]
+ */
+var unionBy = rest(function(arrays) {
+  var iteratee = last(arrays);
+  if (isArrayLikeObject(iteratee)) {
+    iteratee = undefined;
+  }
+  return baseUniq(baseFlatten(arrays, false, true), baseIteratee(iteratee));
+});
+
+export default unionBy;