Javascript中的 setTimeout函数在类中的应用

今天写一个类，在类中使用到了setTimeout函数。

1。 this.timerID = setTimeout("showtime()",1000);

提示：showtime is not defined

２。 this.timerID = setTimeout("this.showtime()",1000);

提示：this.showtime is not a function

３。 this.timerID = setTimeout("this.showtime",1000);

没有错误，但是不执行。

4. 正确的方法：

　　var self = this;

　　this.timerID = setTimeout(function(){self.showtime();},1000);

在方法1 - 3 中找不到showtime()函数。

方法1， showtime指的是一个单独的函数。

方法2, 3，this 指向的是window对象，而不是当前实例对象。

类如下：

// JScript File
function clock()
{
  this.timerID = null;
    this.timerRunning = false;
    this.startHour = 0;
    this.startMinute = 0;
    this.startSecond = 0;
    this.showObj = null;
    
    if(typeof(clock._initialized) == "undefined")
    {
      clock.prototype.init = function(otimerID,timerRunning,startHour,startMinute,startSecond,showObj){
        this.timerID = otimerID;
        this.timerRunning = timerRunning;
        this.startHour = startHour;
        this.startMinute = startMinute;
        this.startSecond = startSecond;
        this.showObj = showObj;
        };
      clock.prototype.toggletimer = function(obj){
          if (this.timerRunning)
          {
            obj.value = "Start Timer";
            this.stopclock ();
          }
          else
          {
            obj.value = "Stop Timer";
            this.startclock ();
          }
        };        
      // stop the clock
      clock.prototype.stopclock = function(){
        if (this.timerRunning)
          clearTimeout(this.timerID);
          this.timerRunning = false;
       };
      // start the clock
      clock.prototype.startclock = function(){
        var now = new Date();
        this.startHour = now.getHours();
        this.startMinute = now.getMinutes();
        this.startSecond = now.getSeconds();
        // Make sure the clock is stopped
        this.stopclock();
        this.showtime();
      };
      // actually display the time
      clock.prototype.showtime = function(){
        var i = 0;
        var strSec = "";
        var now = new Date();
        var hours = now.getHours();
        var minutes = now.getMinutes();
        var seconds = now.getSeconds();
        var timeValue = "" + ((hours >12) ? hours -12 :hours);
        timeValue += ((minutes < 10) ? ":0" : ":") + minutes;
        timeValue += ((seconds < 10) ? ":0" : ":") + seconds;
        timeValue += (hours >= 12) ? " P.M." : " A.M.";
        
        var MinutesElapsing = Math.round(((hours - this.startHour)*60 + (minutes - this.startMinute) + (seconds-this.startSecond)/60)*10)/10;
        this.showObj.value = MinutesElapsing;
        
        for (i = 0; i < seconds-this.startSecond; i++) {
          strSec = strSec + "=";
        };
        window.status = timeValue;
        var self = this;
        this.timerID = setTimeout(function(){self.showtime();},1000);
        this.timerRunning = true;
      };
    }
 clock._initialized = true;
}