下面代码中，setTimeout中前段代码写成w就可以运行，写成"w()"就不能运行的原因是什么？

Question

 <script type="text/javascript">
      window.onload= function w(){
          var time=new Date();
          var year=time.getFullYear();
          var month=time.getMonth()+1;
          var day=time.getDate();
          var week=time.getDay()
          var weekday=new Array();
            weekday[0]="星期日";
            weekday[1]="星期一";
            weekday[2]="星期二";
            weekday[3]="星期三";
            weekday[4]="星期四";
            weekday[5]="星期五";
            weekday[6]="星期六";
          var h=time.getHours();
          var m=time.getMinutes();
          var s=time.getSeconds();
        document.getElementById("first1").innerHTML=year+"年"+month+"月"+day+"日"+weekday[week]+h+":"+m+":"+s;
           setTimeout('w()',500)
        }
         
        </script>

Answer 1

因为是回调函数，所以只要传函数名进去就行了。

Answer 2

‘w()’会认为是一个变量，w()则是运行该函数