js有几个缺陷。combox1.doSelectIdx(-1)我看不用调用了。

答案：function getLeftPostion( theObj )
{
  var pos = 0;
  while ( theObj != null )
  {
    pos += theObj.offsetLeft;
    //get the Object which contain theObj.
    theObj = theObj.offsetParent;
  }
  return pos;
}
function getTopPostion( theObj )
{
  var pos = 0; 
  while ( theObj != null )
  {
    pos += theObj.offsetTop;
    //get the Object which contain theObj.
    theObj = theObj.offsetParent;
  }
  return pos;
}
function checkVersion()
{
  var isBadVersion=true;
  var curVer=navigator.appVersion;
  var pos=parseInt(curVer.indexOf("MSIE"));
  if (pos>=1)
  {
    var intVer=parseInt(curVer.charAt(pos+5));
    if (intVer>=5)
    { isBadVersion=false;}
  }
  if (isBadVersion)
  {
    var msg="This page may not be displayed properly:\n"+
            " This product requires Microsoft Internet Explorer 5 or later browser only.";
    alert(msg); 
  } 
}
//check the browser version
checkVersion();
// the array of comboBoies
theArray = new Array();
function combobox(objId, objHandler)
{
  this.comObj = document.all[objId]; 
  //this.comObj.selectedIndex = -1;  我觉得可以去掉这一句
  this.getValue = getValue; 
  this.doResize = doResize; 
  this.doChange = doChange; 
  this.loseFocus = loseFocus; 
  this.doSelectIdx = doSelectIdx;
  this.doSelectValue = doSelectValue;
  this.focus = focus; 
  var strMsg="";
//------------------------------------------------------------------------------------------------------
// create the text object
//------------------------------------------------------------------------------------------------------
  var txtObjIdName = objId + "_text"; 
  if (document.all[txtObjIdName] != null)
  {
    strMsg="The following id: '" + txtObjIdName +"' is used internally by the Combo Box!\r\n"+
           "Use of this id in your page may cause malfunction. Please use another id for your controls.";
    alert(strMsg);
  }
  var txtInner = "<INPUT type='text' id=" + txtObjIdName + " name=" + txtObjIdName +
                 " onblur='" + objHandler + ".loseFocus()' " + 
                 " style='display: none; position: absolute' value='' >";
  this.comObj.insertAdjacentHTML("afterEnd", txtInner); 
  this.txtObj = document.all[txtObjIdName];
//------------------------------------------------------------------------------------------------------
// end
//------------------------------------------------------------------------------------------------------
  this.beResizing = false; 
  this.doResize(); 
  theArray[theArray.length] = this; 
}
function loseFocus()
{
  var theComObj = this.comObj;
  var theTxtObj = this.txtObj;
  var i;
  theComObj.selectedIndex = -1;
  if (theTxtObj.value == "")
  { return; }
  var optLen = theComObj.options.length;
  for (i=0; i<optLen; i++)
  {
    var comVal = theComObj.options[i].text;
    var txtVal = theTxtObj.value;
    if (comVal == txtVal)
    { theComObj.selectedIndex = i;
      return;
    }
  }
}
function doResize()
{
  if (!this.beResizing)
  {
    this.beResizing = true;
    this.txtObj.style.display="none";
    this.comObj.style.position="static";
    this.txtObj.style.posLeft = getLeftPostion(this.comObj);
    this.txtObj.style.posTop = getTopPostion(this.comObj) + 1;
    this.txtObj.style.posWidth = this.comObj.offsetWidth - 16;
    this.txtObj.style.posHeight = this.comObj.offsetHeight;
    this.comObj.style.position ="absolute";
    this.comObj.style.posLeft = this.txtObj.style.posLeft;
    this.comObj.style.posTop = this.txtObj.style.posTop;
    this.offWidth = this.comObj.offsetWidth;
    var strRect = "rect(0 "+(this.comObj.offsetWidth)+" "+ this.comObj.offsetHeight + 
               " "+(this.txtObj.style.posWidth - 2 )+")";
    this.comObj.style.clip = strRect;
    this.txtObj.style.display="";
    this.txtObj.value=this.comObj.options[this.comObj.selectedIndex].text; //我加了这么一句
    this.beResizing = false;
  }
}
function doChange()
{
  var idx = this.comObj.selectedIndex;
  var opt = this.comObj.options[idx];
  this.txtObj.value = opt.text;
  this.txtObj.focus();
  this.txtObj.select();
  this.comObj.selectedIndex=-1;
}
function getValue()
{ return this.txtObj.value; }
function doSelectIdx(i)
{
  var optLen = this.comObj.options.length;
  if ((i >=0) && (i < optLen))
  { this.comObj.selectedIndex = i;
    this.txtObj.value = this.comObj.options[i].text;
    return;
  }
  this.txtObj.value = "";
}
function doSelectValue(v)
{
 //alert(v)
  var optLen = this.comObj.options.length;
  for (i=0;i<optLen;i++)
  {
 if (this.comObj[i].value== v){
  this.comObj.selectedIndex=i
  this.txtObj.value = this.comObj.options[i].text;
 }
 return;
  }
  
//  this.txtObj.value = "";
}

function focus()
{ this.txtObj.focus(); }
/* resize all combobox when window be resized */
function resetAllSize()
{
  var i; 
  for (i=0; i < theArray.length; i++)
  {
    theArray[i].doResize(); 
  } 
}
 
 
 

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