Writing express.js app. Where do helper methods go?

It really depends of what your helper is doing. If it operates with data which is passed as a parameter to it then you may save it in an external module and use require to access it.

// helpers/FormatString.jsmodule.exports = function(str) {   return str.toUpperCase();}// app.jsvar formatter = require("./helpers/FormatString");

However, if you need to modify the request or the response object then I'll suggest to define it as a middleware. I.e.:

app.use(function(req, res, next) {   // ... do your stuff here});

@Krasimir gave a correct answer. Regarding your question how to deal with asynchronous helper functions I can give you an example (not the usual foo/bar, but one of my own helper functions):

var cached; // as modules can act like singletons I can share values heremodule.exports = function fetchGoogleCerts(options, callback) { // <-- callback    var now = Date.now();    if (ttl > now && cached) {        callback(null, cached); // <-- return with success        return;    }    request({            uri: options.certsUrl || defaultCertsUrl,            strictSSL: true        }, function (err, response, body) {            var error, certs;            if (!err && response.statusCode === 200) {                certs = jsonParse(body); // a local function                if (!certs) {                    callback('parse_error', null); // <-- return an error                    return;                }                cached = certs;                // ... more code                callback(null, cached); // <-- success case            } else {                error = {                    error: err,                    statusCode: response.statusCode                };                log.error(error, 'error fetching google certs');                callback(error); // <-- return an error            }        });    }};

And using the helper:

fetchGoogleCerts = require('./google-certs.js'),module.exports = function fetchCertsAndDecodeIdToken(token, options, callback) {    fetchGoogleCerts(options, function (err, googleCerts) {        if (err) {            callback({                errorCode: codes.io_error,                errorMsg: 'Unable to fetch Google certificates',                error: err            });            return;        }        decodeAndVerifyGoogleIdToken(googleCerts, token, options, callback);    });};

As you can see above, the simple solution is to provide a callback function to your asynchronous helper function.

Of course you can also export an Object which extends EventEmitter, then you might not need a callback function, but register for the events. Here is an Example for a helper which emits events.