limit textbox in javascript

function checkInput(data,obj,max){
var result = true;
if (obj.value.length >= max){
var stripped = obj.value.substring(0, max);
document.getElementById(data).value = stripped;
result = false;
}
if (window.event)
window.event.returnValue = result;
return result;
}

onkeypress="checkInput('des',this,250)