Javascript 同步的XMLHttpRequest最后修改时间，如何让它异步

I found a code that could get the last modified time of a file on the server and have adapted it to my needs, it works fine but I get the warning below and reading about the code seems to be resource intensive for the users computer.
The code needs to react (reload) to a file being modified and print out the time since it was last modified.
I notice my computer is (slightly) sluggish when I have two windows open that use this code, perhaps this is the reason.

我可以做些什么来使其成为异步的呢？

主线程上的同步XMLHttpRequest被否决，因为它对最终用户的体验有不利影响.如需更多帮助，请拨打https://xhr.spec.whatwg.org/.

       <script>
            var pageload = new Date();
            function fetchHeader(url, wch) {
                try {
                    var req=new XMLHttpRequest();
                    req.open("HEAD", url, false);
                    req.send(null);
                    if(req.status== 200){
                        return req.getResponseHeader(wch);
                    }
                    else return false;
                } catch(er) {
                    return er.message;
                }
            }

            window.setInterval(function(){
                var time = Date.parse(fetchHeader("Akut.txt",'Last-Modified'));
                var d = new Date();
                var diffSec = Math.round((d - time) / 1000);
                document.getElementById("time").innerHTML = Math.trunc(diffSec/60) + " minutes"; // time since it was modified
                
                if ((d-pageload)/1000 > 10 && diffSec < 5 ){
                    location.reload();
                }
            }, 2500);
        </script>

I found this thread XMLHttpRequest is deprecated. What to use instead? and I have tried to modify the code but I can't get this code to give me the last modified time.
This just returns NaN.

             function fetchHeader(url, wch) {
                try {
                    var xhr = new XMLHttpRequest()
                    xhr.onreadystatechange = function() {
                        if (this.readyState === this.DONE) {
                            console.log(this.getResponseHeader(wch)); // assumed this would work but it doesn't
                        }
                    }
                    xhr.open("HEAD", url)
                    xhr.send()
                } catch(er) {
                    return er.message;
                }
            }

I need help with making this asynchronous. I have tried to read the xhr link from the warning but I'm not much of a Javascript developer so it's hard for me to find what I need to do.
Can someone point me in the right direction here?