计算AJAX下载速度

Question

I would like to display the download speed in real time as a file is downloaded, however the following code is not working. Speed returns 0.0 for the entire download.

https.get(options, function (update) {
    fileSize = (parseInt(update.headers['content-length']) / 1048576).toFixed(1);
    startTime = (new Date()).getTime();
    endTime = startTime;
    update.on('data', function (chunk) {
        file.write(chunk);
        len += chunk.length;
        fileDownloaded = (len / 1048576).toFixed(1);
        endTime = (new Date()).getTime();
        time = (endTime - startTime) / 1000;
        speed = ((fileSize / time) / (1024 * 1024)).toFixed(2);
        progressPercentage.html(fileDownloaded + ' MB / ' + fileSize + ' MB ' + speed + ' MB/s');
    });
});

Answer 1

I think there is maybe a better solution, but here we go:

var startTime = (new Date()).getTime();
var downloadedLength = 0;

update.on('data', function (chunk) {
    // ...

    downloadedLength += chunk.length;

    var now = (new Date()).getTime();
    var speed = downloadedLength / (now - startTime); //bytes/second
    //speed / 1024 should return in kilobytes

    //...
});

Edit:

Consider using Date.now() instead of (new Date()).getTime(); for better performance

Answer 2

Thought the answer provided by Carlos is great using Performance.now() instead of (new Date()).getTime() would be a better choice.

For reference: https://developer.mozilla.org/en-US/docs/Web/API/Performance/now