Split a growing Express app into modular Router files

app.js: 400 lines, three teammates editing it, constant merge conflicts.

BeginnerRead 5 minDoing: under an hour5 steps

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What you'll have at the end

app.js reduced to route mounting only, with users and products routes each living in their own router file

Before you start

You need

An Express app whose route handlers for two or more resources are still declared directly on the app object in one growing file, with the auth middleware they call already written and working.

Not covered

Splitting further into separate controller and service files, and changing what the existing auth middleware itself checks.

Leans on

Sort the file and stand up the router shells

Map every route to its resource
Give each resource its own router

Fold the middleware in and prove nothing broke

Move shared middleware onto the router
Mount both routers in the main file
Confirm every route still behaves the same

Where to go next

Catching unhandled promise rejections in an async Express route

reach for this once a moved route makes an async database call, since a rejected promise inside a router still crashes the app the same way it did before the split.

Validate and sanitize an incoming request body with a schema

reach for this next if a router's own create or update route still trusts the request body as is, since the split makes it obvious per router which routes take user input.

Checked 25 Aug 2026

Part of the Express.js cookbook