下拉菜单支持输入，并根据输入内容自动定位：

参考：演员发表于 10/23/2001 8:58:16 AM 的文章 “罗亭的可输入下拉框的解密简化版.”，在此特别感谢相关人等。

本文为这个下拉框增加了一点小小的功能：输入能够定位在已有的选择框内。还有一个缺点，各位给改改：输入的时候不能够自动拉开选择框，怎么办？

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;
    this.getValue = getValue;
    this.doResize = doResize;
    this.doChange = doChange;
    this.loseFocus = loseFocus;
    this.doSelectIdx = doSelectIdx;
    this.focus = focus;
    this.keyPress = keyPress;
    this.change = change;

    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()' " + " onkeyup='" + objHandler + ".keyPress()'" +  " onchange='" + objHandler + ".keyPress()'" + " 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.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 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();
    }
}
////////////////定位函数开始，我加的///////////////
function keyPress()
{
    var txtStr;
    var comStr;
    var maxInt = 0;
    var defInt = 0;
    var defJ;
    txtStr = this.txtObj.value;
    //document.form1.test.value=txtStr;
    var j;
    for(j=0;j<this.comObj.options.length;j++)
    {
        comStr = this.comObj.options[j].text;
        var m;
        for(m=0;m<txtStr.length+1;m++)
        {
            if(txtStr.charCodeAt(m) != comStr.charCodeAt(m))
            {
                maxInt = m;
                break;
            }
        }
        if (defInt < maxInt)
        {
            defInt = maxInt;
            defJ = j;
        }
    }
    this.comObj.selectedIndex = defJ;
}