diff --git a/tests/test_jasinta.py b/tests/test_jasinta.py
--- a/tests/test_jasinta.py
+++ b/tests/test_jasinta.py
@@ -16,3 +16,16 @@ document.write(c);"""
 
 		interpret(parse(s))
 		mock_print.assert_called_with(4)
+
+
+class TestAssignment(TestCase):
+	@patch("builtins.print")
+	def test_interpret(self, mock_print):
+		s = """var a=1;
+var b=1;
+a+=b;
+b+=a;
+document.write(b);"""
+
+		interpret(parse(s))
+		mock_print.assert_called_with(3)