`req.next()` was never documented IIRC. A reference to the `next()` function was tacked onto the `req` object to support the deprecated feature of `res()` being a wrapper for calling `next()`, that was for the old route callback signature `function (req, next)`. Since the method that creates `res` object required the `req` object to be passed-in, we tacked the `next()` onto `req.next()`.