function val()
{
var temp=ajaxfun();//ajax function;
}

function ajaxfun()
{
//url,type, data will go here and then
success: function(somedata){
//here i want to return data i.e. somedata
//something like "return somedata" to val();
}
}

<select>
  <option value="v">Volvo</option>
  <option value="s">Saab</option>
  <option value="m">Mercedes</option>
  <option value="a">Audi</option>
</select>

function validateFormOnSubmit(theForm) 
{
vUsername(theForm.userName,document.getElementById('UError'));//this will return true or false depends on error occurrence. 
}


<form name="form" onsubmit="return validateFormOnSubmit(this);">
<input type="text" id="userName" onblur="vUsername(this,UError);">