If in ajax successfull doesnt work

I have a script that will check a file, then the file returns 1 or 0. But even if it returns 1 the script alert the else. Why does this dont work?

When i alert txt it show 0 but when the txt is 1 the alert(txt) gives me 1.

So my question is, why does not hi! comes up when txt is 1?

        $(document).ready(function(){
        setInterval(function(){ 
        //code goes here that will be run every 5 seconds.    
        var txt = '';
        $.ajax({
         type: "POST",
         url: "checkfolder.php",
         success: function(result) {
             txt = result + '';
             if (txt == "1"){
                 alert("Hi!");
             }
             else{
                 alert(txt);
             }

         }
     });
 }, 5000);
});