title: variables not reachable in prototype-methods
body: <p>I can't reach the variables I've declared in the constructor method of my <code>ImageLoaderClass</code> in any prototype method of the Class:</p>
  
  <pre><code>&lt;div id="progress" &gt;&lt;/div&gt;
  &lt;a href="javascript:start();"&gt;Test&lt;/a&gt;
  &lt;script&gt;
  function ImageLoader (url,ziel){
      this.url = url;
      this.ziel=ziel;
      this.request = new XMLHttpRequest();
      this.request.onprogress = this.onProgress;
      this.request.onload = this.onComplete;
      this.request.onerror = this.onError;
      this.request.open('GET', this.url, true);
      this.request.overrideMimeType('text/plain; charset=x-user-defined');
      this.request.send(null);
  }
  ImageLoader.prototype.onProgress=function(event) {
    if (!event.lengthComputable) {
      return;
    }
    alert("url="+this.url);
    this.loaded = event.loaded;
    this.total = event.total;
    this.progress = (this.loaded / this.total).toFixed(2);
    document.querySelector('#progress').textContent = 'Loading... ' + parseInt(this.progress * 100) + ' %';
  }
  ImageLoader.prototype.onComplete=function(event) {
      alert(this.url);
      document.querySelector('#progress').setAttribute('src', this.url);
      console.log('complete', this.url);
  }
  
  ImageLoader.prototype.onError=function(event) {
    console.log('error');
  }
  
  function start(){
      //var request = new XMLHttpRequest();
      var Fab=new ImageLoader('https://placekitten.com/g/2000/2000','#progress');
  }
  &lt;/script&gt;
  </code></pre>