prototypeはchainする

A = function() {}
A.prototype.myfunc = function() {
    alert("A's myfunc")
}

B = function() {}
B.prototype = new A();
B.prototype.myfunc = function() {
    alert("B's myfunc")
}

b1 = new B();
b1.myfunc = function() {
    alert("b1's myfunc")
}
b1.myfunc(); // b1's myfunc

b2 = new B();
b2.myfunc(); // b2's myfunc not found -> B.prototype's myfunc

delete B.prototype.myfunc

b3 = new B();
b3.myfunc(); // b3's myfunc not found -> B.prototype's myfunc not found -> A.prototype's myfunc