How to use?

Three steps are required:

1) Retrieve your .class file from the web, as usual:

function retrieve(url){
  var request = new XMLHttpRequest();
  request.open("GET", url, false);
  request.overrideMimeType("text/plain; charset=x-user-defined");
  request.send();
  if ( (200 !== request.status) && (0 !== request.status) ){
    throw new Error(request.status + " Retrieving " + url); 
  }
  return request;
};

var filename = "<your .class filename url here>";

var file = retrieve(filename).responseText;

2) Create a JD.Stream object from the retrieved file:

var stream = new JD.Stream(file);

3) Create a JD.ClassFile object from the stream:

var classFile = new JD.ClassFile(stream);

What you get?

A JD.ClassFile having all class file components according current specification JSR-202.