I’ve just had to do this for an application of mine where the onbeforeunload handler brings up a prompt if a user leaves a page while a document is in an unsaved state. Python code:
driver.switch_to.alert.accept()
The Java equivalent would be:
driver.switchTo().alert().accept();
If the alert does not exist, the code above will fail with a NoAlertPresentException so there is no need for a separate test to check the existence before accepting the prompt.
I’m running Selenium 2.43.0 but I think this has been doable for a while now.
In cases where I don’t want the prompt to come up at all because that’s not what I’m testing, I run custom JavaScript in the browser to set window.onbeforeunload to null before leaving the page. I put this in the test teardown code