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!rn"+
"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()\'" + " 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.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;
}