Detecting an launching an iOS app, or redirecting to fallback url.

gistfile1.html

<iframe id="i" style="display:none" height=0 width=0></iframe>

<script>
[...]
//// CONFIGURATION
launch_timeout  = 1500;
margin_of_error = 500;
close_delay     = 1000;
close_interval  = 3000;

var i = document.getElementById('i');
launched_at = Date.now();
setTimeout(function() {
	arrived_at = Date.now();
	if (arrived_at - launched_at > launch_timeout + margin_of_error) {
		set_message(MESSAGE_CLEANUP_REQUIRED);
		if (history.length > 1) {
			history.back();
		}
		else {
			setTimeout( function(){ close() }, close_delay);
			setInterval(function(){ close() }, close_interval);
		}
	} else {
		set_message(MESSAGE_FALLING_BACK);
		location.replace(fallback_url);
	}
}, launch_timeout);

i.src = applink;  // Launch!
</script>