javascript json object value returns undefined

I realize there are 100’s of posts about javascript json objects. And all of them imply my code should work, so I am sure I am missing something really stupid. Every attempt to access the json object’s key value results in undefined even though the json.parse works fine.

var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
         if (this.readyState == 4 && this.status == 200) {
            var resp = JSON.parse(this.responseText);
            alert(this.responseText);
            alert(resp.toString());
            alert(resp.exists);
            alert(resp['exists']);
            
         }
    };

This results in the following for alerts:

"{"exists":"True"}"
{"exists":"True"}
undefined
undefined

What incredibly obvious and dumb thing am I missing? I even attempted to use my exact string in w3schools example and it appears to work fine
https://www.w3schools.com/js/tryit.asp?filename=tryjson_object_dot

Thank you in advance.