Getting an alert has the same effect as manually clicking OK. If an alert is generated but you do not get/verify it, the next Selenium action will fail.
NOTE: under Selenium, JavaScript alerts will NOT pop up a visible alert dialog.
NOTE: Selenium does NOT support JavaScript alerts that are generated in a page's onload() event handler. In this case a visible dialog WILL be generated and Selenium will hang until someone manually clicks OK.
The message of the most recent JavaScript alert