[glenjamin]

On this point I don't disagree, for this case you'd expect the data access layer to deal with all the database errors and return something useful to the request layer:

    app.get('/price', function(req, res) {
      products.fetchOne(function(err, product) {
        if (err) {
          res.render('error', { error: err });
        } else {
          res.render('product', { product: product });
        }
      });
    });

[moe]

you'd expect the data access layer to deal with all the database errors and return something useful

There you nailed the problem. It's a constant headache, especially since library authors have different ideas about the format and semantics of "something useful".

Before you know it you have re-invented your own half-baked exception-framework, to normalize/wrap/re-throw those ErrBacks. And then a week later you notice that rollback/retry and event cascades need a whole new level of treatment.

There's a reason why mature event-frameworks such as Twisted have never quite taken over the mainstream. It's sad to see node burn all its powder on a niche programming-style that just doesn't fly for the majority of applications.