prototype properties

gistfile1.js

function X() {
}

Object.defineProperty(X.prototype, '__data',
{enumerable: false, configurable: true, writable: true, value: {x: 0}});

var x1 = new X();
console.log(Object.keys(x1));
console.log(x1.__data);
x1.__data = {x: 1};
console.log(Object.keys(x1));
var x2 = new X();
console.log(x2.__data);
x2.__data = {x: 2};
console.log(Object.keys(x1));
console.log(x1);
console.log(Object.keys(x2));
console.log(x2);