problem.js

var Obj = function() {
  this.func = function() {
    return something.f().then(this._a, this._b); 
  }
  
  this._a = function() {
    //something...(終わったらhoge実行)
  }
  
  this._b = function() {
    //something...(終ったらfuga実行)
  }
}

var o = new Obj();
o.func.then(hoge, fuga);