Resource Timing API JavaScript ping

ping.js

function perf_ping(host, callback, start) {
    if (start == undefined) {
        var perfEntries = performance.getEntries();
        // use the last perf entry to ignore any request preceding
        // this ping() call
        start = 0;
        for (var i = 0; i < perfEntries.length; i++) {
            if (start < perfEntries[i].startTime) {
                start = perfEntries[i].startTime;
            }
        }
        console.log("start: " + start);
    }

    var timer = setTimeout(function() {
        callback('timeout');
    }, TIMEOUT);

    var url = 'http://' + host + '/ping';
    var random_thing = "?" + String(Math.random()).slice(2)

    function ok() {
        clearTimeout(timer);

        var average = 0;
        var count = 0;

        var perfEntries = performance.getEntries();
        for (var i = 0; i < perfEntries.length; i++) {
            var e = perfEntries[i];

            // ignore old entries
            if (e.startTime < start) {
                continue;
            }

            // ignore other stuff
            if (e.name.slice(0, url.length) != url || e.entryType != 'resource') {
                continue;
            }

            var time = e.duration - Math.max(e.requestStart - e.startTime, 0);
            average = (average*count + time) / (count+1);
            count += 1;
        }

        if (count < NPINGS) {
            ping(host, callback, count, start);
            return;
        }

        average -= 2; // average difference to a simple icmp ping
        callback(Math.round(average) + 'ms');
    }

    var img = new Image();
    img.onload = ok;
    img.onerror = ok;
    img.src = url + random_thing;
    callback('...');
}