Introduction:
We can easily create autocomplete textbox using some set array values.Lot of free libraries are available for autocomplete textbox,

Main:

See this below examble,I tried auto complete for some text in no’s 1-10.

<script type=”text/javascript”>
var isOpera = navigator.userAgent.indexOf(“Opera”) > -1;
var isIE = navigator.userAgent.indexOf(“MSIE”) > 1 && !isOpera;
var isMoz = navigator.userAgent.indexOf(“Mozilla/5.”) == 0 && !isOpera;

function MainTextBox (MainTextBox, iStart, iEnd) {

   switch(arguments.length) {
       case 1:
           MainTextBox.select();
           break;

       case 2:
           iEnd = MainTextBox.value.length;
          
           
       case 3:         
           if (isIE) {
               var MainTextBoxRange = MainTextBox.createTextRange();
               MainTextBoxRange.moveStart(“character”, iStart);
               MainTextBoxRange.moveEnd(“character”, -MainTextBox.value.length + iEnd);     
               MainTextBoxRange.select();                                             
           } else if (isMoz){
               MainTextBox.setSelectionRange(iStart, iEnd);
           }                    
   }

   MainTextBox.focus();
}

function AutoReplace (MainTextBox, sText) {

   if (isIE) {
       var MainTextBoxRange = document.selection.createRange();
       MainTextBoxRange.text = sText;
       MainTextBoxRange.collapse(true);
       MainTextBoxRange.select();                                
   } else if (isMoz) {
       var iStart = MainTextBox.selectionStart;
       MainTextBox.value = MainTextBox.value.substring(0, iStart) + sText + MainTextBox.value.substring(MainTextBox.selectionEnd, MainTextBox.value.length);
       MainTextBox.setSelectionRange(iStart + sText.length, iStart + sText.length);
   }

   MainTextBox.focus();
}

function AutoTextMatch (sText, arrValues) {

   for (var i=0; i < arrValues.length; i++) {
       if (arrValues[i].indexOf(sText) == 0) {
           return arrValues[i];
       }
   }

   return null;

}

function Main(MainTextBox, oEvent, arrValues) {

   switch (oEvent.keyCode) {
       case 38: //up arrow 
       case 40: //down arrow
       case 37: //left arrow
       case 39: //right arrow
       case 33: //page up 
       case 34: //page down 
       case 36: //home 
       case 35: //end                 
       case 13: //enter 
       case 9: //tab 
       case 27: //esc 
       case 16: //shift 
       case 17: //ctrl 
       case 18: //alt 
       case 20: //caps lock
       case 8: //backspace 
       case 46: //delete
           return true;
           break;

       default:
           AutoReplace(MainTextBox, String.fromCharCode(isIE ? oEvent.keyCode : oEvent.charCode));
           var iLen = MainTextBox.value.length;

           var sMatch = AutoTextMatch(MainTextBox.value, arrValues);

           if (sMatch != null) {
               MainTextBox.value = sMatch;
               MainTextBox(MainTextBox, iLen, MainTextBox.value.length);
           } 
          
           return false;
   }
}
       </script>
       <script>
               var arrValues = ["one", "MainTextBoxRange", "two", "Three", "Four", "Five", "Six", "Seven","Eight","Nine","Ten"];
       </script>

For Download sample Code, sampleautocompletetextbox

Conclusion:
Hope,this helps,
Happy Coding.