Confused on jquery .ajax .done() function

I think it should be result == 'true' as the result is a data string

I just checked, I am correct, the quotes make it work

PHP:

<?php

if($_POST['username']=='sambob'){echo 'true';}else{echo 'false';}

?>

Javascript

username='sambob';

$(".check").blur(function(){
  $.post("validateUser.php", { username: username })
  .done(function(data) {
     if (data == 'true'){
        alert("User exists");
     }else{
        alert("User doesn't exist"); 
     }
  });
});

json PHP

<?php

if($_POST['username']=='sambob'){echo'{"exists":true}';}
                            else{echo'{"exists":false}';}
?>

json Javascript

$(".check").blur(function(){
   $.post("validateUser.php", { username : username },
   function(user){
      if (user.exists == true){
         alert("User exists");
      }else{
         alert("User doesn't exist");
      }
   }, "json");
});

On your php side, you should echo some json string, for example I'll do like this on the validateUser.php :

//Check your database etc.
$user_exists = array('error'=>false,'user'=>true);
echo json_encode($user_exists);

And than with jQuery :

$.ajax({
    url: "validateUser.php",
    type: "post",
    data: "username= " + username,
    dataType: "json",
  }).done(function(result){
    if (result.error == false){
        //No errors, check user
        if(result.user == true)
            alert("Exists"); //now do some stuff
        else
            alert("User don't exists");
    }else{
      // There is an error
    }
  });