iOS check if an app is installed with a fallback event.

iOS-app-exists-check

function doesAppExist() {
  // try to open iOS application
	document.location = 'customiosappurl://';
	
  // if above failed, nothing happened
  // set a timeout and do something else
	!window.document.webkitHidden && setTimeout(function() {
		setTimeout(function() {				
			window.location = '//apple.itunes.com/whatever'
		}, 100);
	}, 600);
}