javascript valueOf() and toString()

<script type="text/javascript">
function Money(gold, silver, coin)
{
this.gold = gold;
this.silver = silver;
this.coin = coin;
}

Money.parse = function(value)
{
var coin = parseInt(value % 25);
var silver = parseInt(value / 25 % 20);
var gold = parseInt(value / 500);
return new Money(gold, silver, coin);
}

Money.prototype.valueOf = function()
{
return ((this.gold * 20) + this.silver) * 25 + this.coin;
}

Money.prototype.toString = function()
{
return this.gold + "两黄金，" + this.silver + "两白银，" + this.coin + "文钱";
}

var money1 = new Money(5, 6, 1);  //valueOf  (5*20+6)*25+1=2651
var money2 = new Money(2, 4, 6);  //valueOf  (2*20+4)*25+6=1106
var money3 = Money.parse(money1 + money2);// parse(3757) new Money(7,10,7)
alert(money3);
</script>

<script type="text/javascript">
function Money(n)
{
this.n = n;
}
Money.prototype.valueOf = function()
{
return this.n * 2
}
Money.prototype.toString = function()
{
return this.n * 3
}
var m1 = new Money(1); //对象默认返回的是toString()方法
var m2 = new Money(2); //对象默认返回的是toString()方法
m = m1 + m2; //涉及到数学运算，调用其valueOf()方法，所以返回2+4  6
alert(m1);
alert(m2);
alert(m);
</script>

<script type="text/javascript">
var o = {
        x: 8,

        valueOf: function(){
         alert('xxx');
            return this.x + 2;
        },
        toString: function(){
         alert('yyy');
            return this.x.toString();
        }
    },
    result = o < "9"; //数学比较运算，调用valueOf()方法，返回的是 10 < "9" 字符串被转化为数字，为 10 < 9 所以是false
alert(o); //对象默认返回的是toString()方法，所以其值是8，准确的说是字符串"8"

alert(typeof(o.toString())); //返回string
</script>