The title is the error I get. ATM I am upgrading the api to a higher version. Now i got this error. I have tried to fix it but no chance for me. Can someone help?

The code that gives the error is

appendCalls(callList = JSON.parse(xhr.responseText));

The whole function:

function getCallList() {
        var date = formatDate(currentDisplayDate);
        callCounter = 0;
        unansweredCalls = 0;
        
        $('#callCounter').html('');
        $('#spinner').addClass('display');
            
        xhr=new XMLHttpRequest();
        
        xhr.onreadystatechange=function() {
            console.log("sd1"); 
            if (xhr.readyState==4 && xhr.status==200) {
                console.log("sd2"); 
                appendCalls(callList = JSON.parse(xhr.responseText));
                console.log("sd3"); 
            }
        }
    
        xhr.open("GET","/api.php?f=calllist&date="+date,true);
        xhr.send();
        
    }

"appendCalls" is another function. If you need it I can post it.