Dynamically generate textbox and only allow users to input integer in the textbox (char by char parse)?

Could anyone give me some sample codes to show how to dynamically generate textbox and only allow users to input integer in the textbox (char by char parse)?

 In the html page, I have javascript code as follows:

function onlyNumbers(e){

var keynum;
var keychar;var numcheck;
 
if(window.event) ...