Large file upload with WebSocket

In order to serialize this operation you need the server to send you a signal every time a slice is received & written (or an error occurs), this way you could send the next slice in response to the onmessage event, pretty much like this:

function Uploader(url, file) {
    var fs = new FileSlicer(file);
    var socket = new WebSocket(url);

    socket.onopen = function() {
       socket.send(fs.getNextSlice());
    }
    socket.onmessage = function(ms){
        if(ms.data=="ok"){
           fs.slices--;
           if(fs.slices>0) socket.send(fs.getNextSlice());
        }else{
           // handle the error code here.
        }
    }
}

I believe the send() method is asynchronous which is why it will return immediately. To make it queue, you'd need the server to send a message back to the client after each slice is uploaded; the client can then decide whether it needs to send the next slice or a "upload complete" message back to the server.

This sort of thing would probably be easier using XMLHttpRequest(2); it has callback support built-in and is also more widely supported than the WebSocket API.

Use web workers for large files processing instead doing it in main thread and upload chunks of file data using file.slice().

This article helps you to handle large files in workers. change XHR send to Websocket in main thread.

//Messages from worker
function onmessage(blobOrFile) {
 ws.send(blobOrFile);
}

//construct file on server side based on blob or chunk information.