[This doesn't work](http://rollupjs.org/?shareable=JTdCJTIyb3B0aW9ucyUyMiUzQSU3QiUyMmZvcm1hdCUyMiUzQSUyMmNqcyUyMiUyQyUyMm1vZHVsZU5hbWUlMjIlM0ElMjJteUJ1bmRsZSUyMiUyQyUyMmdsb2JhbHMlMjIlM0ElN0IlN0QlN0QlMkMlMjJtb2R1bGVzJTIyJTNBJTVCJTdCJTIybmFtZSUyMiUzQSUyMm1haW4uanMlMjIlMkMlMjJjb2RlJTIyJTNBJTIydmFyJTIwX3dpbmRvdyUyMCUzRCUyMHdpbmRvdyUzQiU1Q24lNUNud2luZG93LmZvbyUyMCUzRCUyMDElM0IlNUNuX3dpbmRvdy5iYXIlMjAlM0QlMjAyJTNCJTIyJTdEJTJDJTdCJTIybmFtZSUyMiUzQSUyMmZvby5qcyUyMiUyQyUyMmNvZGUlMjIlM0ElMjIlMjIlN0QlNUQlN0Q=): ``` js var _window = window; window.foo = 1; _window.bar = 2; ``` Only the `window.foo = 1` side-effect is preserved.