问异步代码返回错误变量的问题

EN

function makePlaceRequest(XMLaddress, callback) {
        httpRequest = new XMLHttpRequest();
        // alert('Starting XML read');
        if (!httpRequest) {
            alert('Warning :( Cannot create an XMLHTTP instance');
            return false;
        }

        // httpRequest.onreadystatechange = getPlace;

        // State changes

        httpRequest.onreadystatechange = function() {
            try {
                if (httpRequest.readyState === XMLHttpRequest.DONE) {
                    if (httpRequest.status === 200) {
                        var xmldoc = httpRequest.responseXML;
                        var root_node = xmldoc.getElementsByTagName('placecode').item(0);
                        PC = root_node.firstChild.data;
                        // callback();
                    }
                    else {
                        alert('There was a problem with the request.');
                    }
                }
            }
            catch( e ) {
                alert('Caught Exception: ' + e.description);
            }
        }

        httpRequest.open('GET', XMLaddress);
        // httpRequest.open('GET', '/xml/Places.xml');
        httpRequest.send();

        callback();
    }

    makePlaceRequest('/xml/Places.xml', function() {
        alert(PC);
        alert('Completed XML read');
        alert(PC);
    });