check.js

  checkShit = function(url) {
    var request = new XMLHttpRequest();
    request.open('HEAD', url, true);

    request.onload = function() {
      if (request.status >= 200 && request.status < 400) {
        console.log('it\'s there');
      } else {
        console.log('it\'s not there');
      }
    };
    request.send();
  }
  
  checkShit('folder/file.ext');