function Base(){
}

Base.prototype = {x:10, y:[]}

function A(){}
A.prototype = new Base();
var a = new A();
a.y.push("first");
a.x = 9;

console.log(a.x);
console.log(a.y);


function B(){}
B.prototype = new Base();
var b = new B();
b.y.push("second");
b.x = 11;

console.log(a.x);
console.log(a.y);

9
["first"]
9
["first", "second"]