How to get the type of the returned object from .POST ajax call?

Im using ajax .POST to run php script that suppose to return true/false. Im returning true/false using "echo" in my script. But after i get the result back to JS i compare the received text in an IF statement that never works! here is my code

 $.ajax({ url: 'admin_checkuser.php',
      data: {action: window.myId},
      type: 'post',
      success: function(output) {
 
 if (output == "true"){
 

and here is the php script that being called

include_once('class.MySQL.php');
 
 $userid = $_POST['action'];
 
 $oMySQL = new MySQL();
 $query = "Select * FROM videotable WHERE uid = '$userid'";
 $oMySQL->ExecuteSQL($query);
 $bb = $oMySQL->iRecords;
 $aa = $oMySQL->aResult;
 
 
 if ($bb == 0){
 $query = "INSERT INTO videotable VALUES ('','$userid','true')";
 $oMySQL->ExecuteSQL($query);
 echo 'true';
 exit();
 
 }else{
 $sharing = mysql_result($aa,0,"share");
 echo $sharing;
 exit();
 
 }
 

I made sure that i receive true\false by doing "alert(output)" and it always displays true\false so i really dont understand why my IF statement fails even when alert(output) shows true

Thanks in advance!

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