JS报错：Uncaught TypeError: Object [object ?报错

正常代码：

<script type="text/javascript"> function ShaBi(name) { this.name = name; }

ShaBi.prototype.startSb = function() { var $timer = setInterval(this.hehe,50); }

ShaBi.prototype.hehe = function() { alert("!"); }

var sb = new ShaBi("sb"); sb.startSb(); </script>

错误代码：

<script type="text/javascript">
function ShaBi(name)
{
    this.name = name;
}

ShaBi.prototype.startSb = function()
{
    var $timer = setInterval(this.hehe,50);
}

ShaBi.prototype.hehe = function()
{
    this.haha();
}

ShaBi.prototype.haha = function()
{
    alert("!");
}

var sb = new ShaBi("sb");
sb.startSb();
</script>

错误：

Uncaught TypeError: Object [object global] has no method 'haha'

没搞明白这是什么意思= =