js笔试题

var A = function () {};

A.prototype.n = 1;

var b = new A();

A.prototype = {

　　n:2,

　　m:3

}

var c =new A();

console.log(b.n);//1

console.log(b.m)//undefined

console.log(c.n);//2

console.log(c.m)//3