Successful handling of the prompt requires prior execution of the answerOnNextPrompt command. If a prompt is generated but you do not get/verify it, the next Selenium action will fail.
NOTE: under Selenium, JavaScript prompts will NOT pop up a visible dialog.
NOTE: Selenium does NOT support JavaScript prompts 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 question prompt