Mounting routers with app.use in Express - Step-by-Step Execution

Concept Flow - Mounting routers with app.use

Create Express app

↓

Create Router

↓

Define routes on Router

↓

Mount Router on app with app.use(path, router)

↓

Incoming request to app

↓

Check if request URL matches mount path

Yes↓

Pass request to Router

↓

Router handles route

↓

Send response

↓

Request complete

This flow shows how an Express app creates a router, mounts it with app.use, and how requests matching the mount path are handled by the router.

Execution Sample

Express

const express = require('express');
const app = express();
const router = express.Router();
router.get('/hello', (req, res) => res.send('Hello from router'));
app.use('/api', router);

This code creates an Express app, a router with a /hello route, and mounts the router at /api.

Execution Table

Step	Action	Request URL	Match Mount Path?	Router Route Matched?	Response Sent
1	Request arrives at app	/api/hello	Yes (/api)	Yes (/hello)	Hello from router
2	Request arrives at app	/api/goodbye	Yes (/api)	No	404 Not Found
3	Request arrives at app	/hello	No	N/A	404 Not Found

💡 Requests not matching the mount path or router routes result in 404 responses.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
app	Express app instance	Receives /api/hello	Receives /api/goodbye	Receives /hello
router	Express router instance	Handles /hello route	No matching route	No matching route
request URL	N/A	/api/hello	/api/goodbye	/hello
response	N/A	Sent 'Hello from router'	Sent 404	Sent 404

Key Moments - 2 Insights

Why does a request to /hello not get handled by the router mounted at /api?

Because the router is mounted at /api, only requests starting with /api match the mount path (see execution_table row 3). Requests to /hello do not match and are not passed to the router.

What happens if the router does not have a route matching the request after matching the mount path?

If the router has no matching route (see execution_table row 2), Express returns a 404 Not Found response because no handler was found.

Visual Quiz - 3 Questions

Look at the execution_table, what response is sent when the request URL is /api/hello?

A "Hello from router"

B 404 Not Found

C No response sent

D "Hello from app"

💡 Hint

Check execution_table row 1 under 'Response Sent'

At which step does the request URL not match the mount path?

A Step 1

B Step 2

C Step 3

D None

💡 Hint

Look at 'Match Mount Path?' column in execution_table

If the router was mounted at '/' instead of '/api', what would happen to the request at step 3?

A It would be handled by the router if a matching route exists

B It would still return 404 immediately

C The app would crash

D The request would be ignored

💡 Hint

Consider how mounting at '/' matches all paths, see concept_flow

Concept Snapshot

Mount routers with app.use(path, router)
- Create router with express.Router()
- Define routes on router
- Mount router on app with app.use('/path', router)
- Requests starting with '/path' go to router
- Router handles routes relative to mount path
- Non-matching requests skip router

Full Transcript

This lesson shows how to mount routers in Express using app.use. First, you create an Express app and a router. Then you define routes on the router. Next, you mount the router on the app at a specific path using app.use. When a request comes in, Express checks if the URL starts with the mount path. If yes, it passes the request to the router. The router then tries to match the rest of the URL to its routes. If a route matches, it handles the request and sends a response. If no route matches, Express sends a 404 Not Found. Requests that do not start with the mount path do not reach the router and also get 404 if no other handler exists. This way, routers help organize routes under specific URL prefixes.