As others have already shared, app.set('config', config) is great for this. I just wanted to add something that I didn't see in existing answers that is quite important. A Node.js instance is shared across all requests, so while it may be very practical to share some config or router object globally, storing runtime data globally will be available across requests and users. Consider this very simple example:
var express = require('express');
var app = express();
app.get('/foo', function(req, res) {
app.set('message', "Welcome to foo!");
res.send(app.get('message'));
});
app.get('/bar', function(req, res) {
app.set('message', "Welcome to bar!");
// some long running async function
var foo = function() {
res.send(app.get('message'));
};
setTimeout(foo, 1000);
});
app.listen(3000);
If you visit /bar and another request hits /foo, your message will be "Welcome to foo!". This is a silly example, but it gets the point across.
There are some interesting points about this at Why do different node.js sessions share variables?.