测试题

  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, b.m, c.n, c.m) //1 undefined 2 3

  var F = function(){};
  Object.prototype.a = function(){
    console.log('a()')
  };
  Function.prototype.b = function(){
    console.log('b()')
  };
  var f = new F();
  f.a() //a()
  f.b() //f.b is not a function
  F.a() //a()
  F.b() //b()