无法使用javascript打印AJAX返回的数组

function getWriters(cat, lev, id)   
{
    if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    }
    else { // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200)

        {

            document.getElementById("writer").innerHTML = xmlhttp.responseText;
            var writer = eval(xmlhttp.responseText);
            document.write(writer)
        }


    }

    xmlhttp.open("GET", "order.php?op=2&id=0&cat=" + cat + "&lev=" + lev, true);
    xmlhttp.send();


}
function getWriters(cat, lev, id)   
{
    if (