node.js instead of redirect show alert message

I have written a node.js code which is basically uploading a file to desired folder. the code is as shown below:

app.post('/upload', function(req, res) {
    req.pipe(req.busboy);
    req.busboy.on('file', function(fieldname, file, filename) {
        var fstream = fs.createWriteStream('./python_codes/' + filename); 
        file.pipe(fstream);
        fstream.on('close', function () {
            res.send('upload succeeded!');
            console.log("fileuploaded")
        });
    });
});

but on clicking the upload button on client side it is redirecting to new page. instead of redirecting I want to show a alert message. Thanks in advance!

via nishant kumar