script404checker.js

//loop through all <script> tags and fire async XMLHttpRequest to report 404's
var scripts = document.getElementsByTagName("script");
var i;
for (i = 0; i < scripts.length; i++) {
  if (scripts[i].src === "") continue;
  var xhr = new XMLHttpRequest();
  //closure on url so it's available when onreadystatechange fires later
  //remember js needs a function block to closure
  xhr.onreadystatechange = (function(url) {
    return function() {
      if (this.readyState === 4 && this.status === 404) {
        console.error("bad script url: " + url);
        var h3 = document.createElement("h3");
        h3.innerHTML = "bad url: " + url;
        document.body.insertBefore(h3, document.body.firstChild);
      }
    };
  })(scripts[i].src);
  xhr.open("GET", scripts[i].src, true);
  xhr.send(null);
}