function getHTTPObject(){
   if (window.ActiveXObject) return new ActiveXObject("Microsoft.XMLHTTP");
   else if (window.XMLHttpRequest) return new XMLHttpRequest();
   else {
      alert("Your browser does not support AJAX.");
      return null;
   }
}   
 
// Change the value of the outputText field
function setOutputdiv(){
	if(httpObject.readyState == 4){
		document.getElementById('divusername').innerHTML = httpObject.responseText;

    }

}

 
// Implement business logic    
function doWork(){    
    httpObject = getHTTPObject();
    if (httpObject != null) {
        httpObject.open("GET", "check.php?check=username&text="
                        +document.getElementById('username').value, true);
        httpObject.send(null); 
        httpObject.onreadystatechange = setOutputdiv;
    }
}

 
var httpObject = null;

<input type="text"  onkeyup="doWork();" name="username" id="username">

	<div id="divusername"></div>

<input type="text"  onkeyup="doWork('username');" name="username" id="username">

	<div id="divusername"></div>

function doWork(ajaxtarget){    
    httpObject = getHTTPObject();
    if (httpObject != null) {
        httpObject.open("GET", "check.php?check=" + ajaxtarget + "&text="
                        +document.getElementById(ajaxtarget).value, true);
        httpObject.send(null); 
        httpObject.onreadystatechange = setOutputdiv;
    }
}

function setOutputdiv(ajaxtargetid){
	if(httpObject.readyState == 4){
		document.getElementById(ajaxtargetid).innerHTML = httpObject.responseText;

    }

}

function doWork(ajaxtarget){    
    httpObject = getHTTPObject();
    if (httpObject != null) {
        httpObject.open("GET", "check.php?check=" + ajaxtarget + "&text="
                        +document.getElementById(ajaxtarget).value, true);
        httpObject.send(null); 
        httpObject.onreadystatechange = setOutputdiv("div" + ajaxtarget);
    }
}

function doWork(ajaxtarget){    
    httpObject = getHTTPObject();
    if (httpObject != null) {
        httpObject.open("GET", "check.php?check=" + ajaxtarget + "&text="
                        +document.getElementById(ajaxtarget).value, true);
		httpObject.onreadystatechange = function(){
			if(httpObject.readyState == 4){
				document.getElementById('div_' + ajaxtarget).innerHTML = httpObject.responseText;
			}
		}
		httpObject.send(null);
    }
}