Sample connect/express middleware to add a digest to all file uploads. Useful for change detection, that kind of thing. Also an example of calculating a hash (SHA1 or MD5) on a file.

digestFile.js

var crypto = require('crypto'),
    fs = require('fs');

/**
 * Calculate the digest of a given file as a hex string.
 *
 * @param filename Name of the file to read
 * @param {String} [algo='sha1'] Hash algorithm (e.g. 'md5' or 'sha1')
 * @param {function(Exception,String) cb Callback which is invoked with (err,hash)
 */
function digestFile(filename, algo, cb) {
    if(typeof(algo) === 'function') {
        cb = algo;
    }
    if(typeof(algo) !== 'string') {
        algo = 'sha1';
    }

    var hash = crypto.createHash(algo);

    var s = fs.ReadStream(filename);
    s.on('data', function(d) {
        hash.update(d);
    });

    s.on('end', function() {
        cb(null, hash.digest('hex'));
    });

    s.on('error', function(err) {
        cb(err);
    })
}
exports.digestFile = digestFile;

/**
 * Return middleware that will add the given hash
 * to all file uploads.  Assumes file uploads are
 * being processed by formidable or the connect body parser.
 *
 * @param {String} [algo='sha1'] Hash algorithm to use, e.g. 'md5' or 'sha1'
 */
exports.middleware = function digestFileUploadsMiddleware(algo) {
    return function digestFileUploads(req, res, next) {
        var keys = req.files && Object.keys(req.files);
        if(keys && keys.length) {
            var pending=0;
            keys.forEach(function digestFileUpload(k) {
                var f = req.files[k];
                pending++;
                digestFile(f.path, algo, function(err, digest) {
                    if(err === null) {
                        f[algo] = digest;
                    }
                    pending--;
                    if(pending === 0) {
                        next();
                    }
                })
            });
        } else {
            next();
        }
    }
}