Display the rerun value in JavaScript

Question

I have the following:

var items = [];
 var myfunc= function ()
    {
        $.getJSON("a.txt", function (data1) {
            for (key in data1) {
                items.push(key);
            }
        });
        return items[0] + " test";
        }

with alert(myfunc()), I can't see the first element of my items Array. I declared that to be global but i get this: "undefined test" what could be the reason? Thanks