javascript 上传进度条

function upload() {
                var formData = new FormData(document.getElementById("inputForm"));
                //创建xhr
                var xhr = new XMLHttpRequest();
                var url = "save";
                xhr.open("POST", url, true);
                xhr.onreadystatechange = function () {
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        var result = xhr.responseText;
                        document.write(result);
                    }
                    if(xhr.readyState == 4 && xhr.status == 500){
                        var result = xhr.responseText;
                        document.write(result);
                    }
                };
                //进度条部分
                xhr.upload.onprogress = progressFunction;//【上传进度调用方法实现】
                xhr.send(formData);
            }
            function progressFunction(evt){
                if (evt.lengthComputable) {//
                    loading('正在上传，请您等待...' + Math.round(evt.loaded / evt.total * 100) + "%");
                }
            }