HTML5 带进度条的异步文件上传原理

<form >
    <label for="fileToUpload">Select a File to Upload</label>
    <input type="file" name="fileToUpload" />
</div>
    <div ></div>
    <div ></div>
    <div ></div>
    <div class="row">
        <input type="button" onclick="uploadFile()" value="Upload" />
    </div>
    <div ></div>
</form>

function fileSelected() {
            var file = document.getElementById('fileToUpload').files[0];
            if (file) {
                var fileSize = 0;
                if (file.size > 1024 * 1024)
                    fileSize = (Math.round(file.size * 100 / (1024 * 1024)) / 100).toString() + 'MB';
                else fileSize = (Math.round(file.size * 100 / 1024) / 100).toString() + 'KB';
                document.getElementById('fileName').innerHTML = 'Name: ' + file.name;
                document.getElementById('fileSize').innerHTML = 'Size: ' + fileSize;
                document.getElementById('fileType').innerHTML = 'Type: ' + file.type;
            }
        }
        function uploadFile() {
            var fd = new FormData();
            fd.append("fileToUpload", document.getElementById('fileToUpload').files[0]);
            var xhr = new XMLHttpRequest();
            xhr.upload.addEventListener("progress", uploadProgress, false);
            xhr.addEventListener("load", uploadComplete, false);
            xhr.addEventListener("error", uploadFailed, false);
            xhr.addEventListener("abort", uploadCanceled, false);
            xhr.open("POST", "/file-upload");
            xhr.send(fd);
        }
        function uploadProgress(evt) {
            if (evt.lengthComputable) {
                var percentComplete = Math.round(evt.loaded * 100 / evt.total);
                document.getElementById('progressNumber').innerHTML = percentComplete.toString() + '%';
            } else {
                document.getElementById('progressNumber').innerHTML = 'unable to compute';
            }
        }
        function uploadComplete(evt) {
            /* This event is raised when the server send back a response */
            alert(evt.target.responseText);
        }
        function uploadFailed(evt) {
            alert("There was an error attempting to upload the file.");
        }
        function uploadCanceled(evt) {
            alert("The upload has been canceled by the user or the browser dropped the connection.");
        }

/* @express 4.X
express install
npm install
npm install multer --save
var multer = require ( 'multer' );
app.use(multer({ dest: './file-upload/' }));
*/

var express = require("express"),
    app = express(),
    server = require("http").createServer(app),
    fs = require("fs");
app.listen(6688);
app.use(express.bodyParser({uploadDir: './upload-file'}));
app.use(express.static(__dirname + '/static'));
app.get('/', function(req, res) {
    fs.readFile(__dirname+'/index.html', function(err, data) {
        if (err) {
            res.writeHeader(500);
            res.end('index.html error');
        }
        res.writeHeader(200);
        res.end(data);
    })
});
app.post('/file-upload', function(req, res) {
    res.send('ok');
})