detect if your native app is installed from your web site code example

Example 1: how to check if user has installed pwa

if (window.matchMedia('(display-mode: standalone)').matches) {  
    // do things here
}

Example 2: is it possible to check with my website if an app is installed?

var now = new Date().valueOf();
setTimeout(function () {
    if (new Date().valueOf() - now > 100) return;
    window.location = "https://itunes.apple.com/appdir";
}, 25);
window.location = "appname://";