k.js

function Base(x) {
  // override properties in prototype if necessary
  if (arguments.length == 1) {
    this._x = x;
  }
}

Base.prototype = {
  _x: [],
  push: function(v) { this._x.push(v) }
};

function Derived(x) {
  // override properties in prototype if necessary
  if (arguments.length == 1) {
    Base.call(this, x);
  }
}

Derived.prototype = new Base([]); // default value of _x is [] for Derived

var a = new Derived(), b = new Derived();

// 両方とも同じDerived.prototype._xに対して操作してしまう！

a.push(10);
b.push(20);

console.log(a.__proto__); // { _x: [ 10, 20 ] }
console.log(b.__proto__); // { _x: [ 10, 20 ] }