gistfile1.js

var myAPI = {
    doSomeShit: function (cb) {
        // Need to preserve the binding of cb, 
        // but you can't do cb.this - there's no way to find out what it's bound to
        cb.call(cb.this, 'arg1', 'arg2')
    }
};

function MyClass() {
    this.shitIsDone = false;
}

MyClass.prototype.foo = function () {
    myAPI.doSomeShit(function () {
        this.shitIsDone = true;
    }.bind(this))
}

var myInstance = new MyClass();
myInstance.foo();

doSomeShit: function(cb) {
  cb.prototype.returnThis = function() { return this; };

  cb.call(cb.returnThis(), 'arg1', 'arg2');
}