Aside from alert messages, there are prompt messages that ask users to input their answer into a single text field. To handle this kind of dialog box from web pages, PhantomJS allows us to handle and answer back to this dialog prompts using the onPrompt callback.

page.onPrompt = function(message, defaultVal) {
  if (message === 'How old are you?') {
    return '18';
  }
  
  return defaultVal;
};

The onPrompt function callback receives two parameters. The first parameter is the message to be displayed in the prompt dialog; this normally asks a question back to the user. The second parameter is the default value or answer.