wp-calypso/client/server/isomorphic-routing/README.md

Isomorphic Routing Middleware Adapters

index.js

This module provides functions to adapt page.js-style middleware (with ( context, next ) signature) to work with express.js (( req, res, next )), so client middleware satisfying a couple of constraints can be re-used on the server side.

For more information, see Isomorphic Routing docs.