Formalize which value is picked by _.unionBy and _.unionWith to pick the result from the first array in which it occurs.

lodash.js

@@ -7789,7 +7789,8 @@
     /**
      * 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:
+     * which uniqueness is computed. Result values are chosen from the first
+     * array in which the value occurs. The iteratee is invoked with one argument:
      * (value).
      *
      * @static
@@ -7819,7 +7820,8 @@
 
     /**
      * This method is like `_.union` except that it accepts `comparator` which
-     * is invoked to compare elements of `arrays`. The comparator is invoked
+     * is invoked to compare elements of `arrays`. Result values are chosen from
+     * the first array in which the value occurs. The comparator is invoked
      * with two arguments: (arrVal, othVal).
      *
      * @static

test/test.js

@@ -24541,6 +24541,13 @@
 
       assert.deepEqual(args, [2.1]);
     });
+
+    QUnit.test('should output values from the first possible array', function(assert) {
+      assert.expect(1);
+
+      var actual = _.unionBy([{ 'x': 1, 'y': 1 }], [{ 'x': 1, 'y': 2 }], 'x');
+      assert.deepEqual(actual, [{ 'x': 1, 'y': 1 }]);
+    });
   }());
 
   /*--------------------------------------------------------------------------*/
@@ -24557,6 +24564,19 @@
 
       assert.deepEqual(actual, [objects[0], objects[1], others[0]]);
     });
+
+    QUnit.test('should output values from the first possible array', function(assert) {
+      assert.expect(1);
+
+      var objects = [{ 'x': 1, 'y': 1 }],
+          others = [{ 'x': 1, 'y': 2 }];
+
+      var actual = _.unionWith(objects, others, function(a, b) {
+        return a.x == b.x;
+      });
+
+      assert.deepEqual(actual, [{ 'x': 1, 'y': 1 }]);
+    });
   }());
 
   /*--------------------------------------------------------------------------*/