JSFiddle - React, Tailwind, and code Playground

JavaScript

var url = "/echo/xml/";

function fileExists(url){
    var http = new XMLHttpRequest();
    http.open('HEAD',url,true);
    http.send();
    return http.status != 404;
}

console.log(fileExists(url));
console.log(fileExists(url + "dummy"));

function fileExistsAsync(url, callback){
    var http = new XMLHttpRequest();
    http.open('HEAD',url,true);
    http.send();
    http.onreadystatechange = function () {
    	if (http.readyState === 4) {
        	callback(http.status != 404);
        }
    }
}

fileExistsAsync(url, function (exists) {
	console.log("async " + exists);
})

fileExistsAsync(url + "dummy", function (exists) {
	console.log("async " + exists);
})