Simple `__proto__` based Array sandbox

proto-array-sandbox.js

var sb = {
  Array: function() {
    var result = Array.apply(null, arguments);
    result.__proto__ = sb.Array.prototype;
    return result;
  }
};
sb.Array.prototype  = [];
sb.Array.prototype.last = function () { return this[this.length -1]; };

// Example usage:

var a = sb.Array('a','b','c');
a[3] = 'd';
a.length; // 4
a.last(); // "d"

var b = sb.Array(10);
b.length; // 10