How can I get the progress of a downloading ?

<script> tags only fire load and error events; they do not fire progress events. However, in modern browsers, Ajax requests do support progress events. You can load your script content and monitor progress through Ajax, and then place the script contents into a new <script> element when the load completes:

var req = new XMLHttpRequest();

// report progress events
req.addEventListener("progress", function(event) {
    if (event.lengthComputable) {
        var percentComplete = event.loaded / event.total;
        // ...
    } else {
        // Unable to compute progress information since the total size is unknown
    }
}, false);

// load responseText into a new script element
req.addEventListener("load", function(event) {
    var e = event.target;
    var s = document.createElement("script");
    s.innerHTML = e.responseText;
    // or: s[s.innerText!=undefined?"innerText":"textContent"] = e.responseText
    document.documentElement.appendChild(s);

    s.addEventListener("load", function() {
        // this runs after the new script has been executed...
    });
}, false);

req.open("GET", "foo.js");
req.send();

For older browsers that don’t support Ajax progress, you can build your progress-reporting UI to show a loading bar only after the first progress event (or otherwise, show a generic spinner if no progress events ever fire).