bind-sample.js

var hoge = {
  num: 10,
  twi: function(n, cb){
    cb(n * 2);
  },
  add: function(a, cb){
    this.twi(a, function(ans) {
      cb(ans + this.num);      
    }.bind(this));
  }
}

hoge.add(10, function(ans){
  console.log(ans)
});