That's the easy way to do it
If you use a library, like Sinon.JS, you can do
The upshot of using a library is
1) You get a richer interface for behavior verification (i.e. was it called? how many times? what arguments did it receive? and so on)
2) Sinon sandboxes the test and reverts the original method after the test has run
Beware that IE not always allow you to override host methods like this. I don't remember if it screams when overriding alert, but you'll notice soon enough