Javascript GM_xmlhttpRequest change var

Question

I tried to do that :

var a = []; // Global array
GM_xmlhttpRequest({
      method: "GET",
      url: 'http://server.com/page.php',
      onload: function (response) {

        a.push(response.responseText);
      }
});

alert(a);

But "a" is empty, how can i push into "a" array inside onload function? Thank you in advance.