foo.js

function Foo( callback ) {
  this.callback = callback;
}

Foo.prototype.Bar = function() {
  xhr = new XMLHttpRequest();

  xhr.onreadystatechange = function() {
    console.log(this.callback);
    // => undefined
  }

  xhr.onreadstatechange = function( this ) {
    console.log(this.callback);
    // => Uncaught TypeError: Undefined is not a function.
  }

  xhr.onreadystatechange = function() {
    console.log(Foo.callback());
    // => Uncaught TypeError: Undefined is not a function.
  }

  // Hmmm....
  // How can I get Foo's calback argument?
}