親子丼.js

function Parent () {
  this._state = [];
}

Parent.prototype.foo = function (v) {
  this._state.push(v);
};

Parent.prototype.bar = function () {
  return this._state.join(',');
};

Parent.prototype.spawn = function () {
  return new Child(this.foo.bind(this));
};

function Child (foo) {
  this.foo = foo;
}

var p = new Parent();
var c = p.spawn();
c.foo(1);
c.foo(2);
console.log(p.bar());