Detect if the client's router is a FritzBox

is-fritzbox.js

function isFritzBox(){
    return new Promise(resolve =>{
        let img = document.createElement('img');
        img.onload = _ => resolve(true);
        img.onerror = _ => resolve(false);
        img.src = 'http://fritz.box/favicon.ico';
    });
}