how to use ajax in magento module

first create a controller in controller folder of your module

and insert this script in phtml file where you want get result.

<script>
function loadXMLDoc(cid)
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
   
  }
 
xmlhttp.open("GET","getallcustomer/ajax/index?a="+cid,true);
xmlhttp.send();
}
</script>
//"getallcustomer" :- is module name.
//"ajax":- is controller name (controller file name).
//"index":- is action of controller (function indexAction();)

<div id="myDiv">
ajax responce text print here.
</div>