alert shows all error in javascript

<script type="text/javascript">
function Validateform()
{
var name=document.getElementById("name_surname").value;
var mail=document.getElementById("mail").value;
var textme=document.getElementById("textme").value;

var atpos = mail.indexOf("@");
var dotpos = mail.lastIndexOf(".");
var str="";
flag=true;

if(name=='')
{
str="Please enter your Name\n";
document.getElementById("name_surname").focus();
flag=false;
}
if(mail=='')
{
str+="Please enter your Email id\n";
document.getElementById("mail").focus();
flag=false;
}
if (atpos < 1 || ( dotpos - atpos < 2 ))
   {
str+="Please enter correct email ID\n";
   document.getElementById("mail").focus();
      flag=false;
   }
if(textme=='')
{
str+="Please enter the Subject\n";
document.getElementById("textme").focus();
flag=false;
}  
if(flag==false)
{
alert(str);
return false;
}
else
return true;


}
</script>