Bump to v4.0.0.

unionBy.js

@@ -0,0 +1,35 @@
+define(['./internal/baseFlatten', './internal/baseIteratee', './internal/baseUniq', './isArrayLikeObject', './last', './rest'], function(baseFlatten, baseIteratee, baseUniq, isArrayLikeObject, last, rest) {
+
+  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
+  var undefined;
+
+  /**
+   * 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));
+  });
+
+  return unionBy;
+});