Simple app

node.js

$("#send").click(function(e) {
      // convert canvas to data url
      var img = canvas.toDataURL();
      // make request to server
      $.post("/", {img: img, n: n}, function() {
            // when request is finished, redirect to homepage
            window.location.replace("/");
        })
        return false;
});

app.post('/', function(req, res) {
    // extract arguments from request
    var img = req.body.img
      , n = req.body.n;
 
    // extract image data from img
    var data = img.replace(/^data:image\/\w+;base64,/, "");
    // write image to a binary file
    var buf = new Buffer(data, 'base64');
    var f = "numbers/" + uuid.v1() + "_digit_" + n + ".png";
    fs.writeFile(f, buf);
    // redirect the user to the homepage (gives them a new number to input)
    res.redirect("/");
});