Virtual path prefixes in Express - Step-by-Step Execution

Concept Flow - Virtual path prefixes

Define app

↓

Create router

↓

Attach router with prefix

↓

Request to prefixed path

↓

Router handles request

↓

Send response

This flow shows how Express uses a virtual path prefix to route requests through a router mounted at a specific path.

Execution Sample

Express

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

This code creates a router and mounts it on the app at the '/prefix' path, so requests to '/prefix' go to the router.

Execution Table

Step	Request URL	Router Mounted At	Match?	Handler Called	Response Sent
1	/prefix	/prefix	Yes	router.get('/')	Hello from router
2	/prefix/	/prefix	Yes	router.get('/')	Hello from router
3	/prefix/extra	/prefix	Yes	No handler	No response from router
4	/other	/prefix	No	No handler	No response from router

💡 Requests not starting with '/prefix' do not match the router and are not handled by it.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Request URL	/prefix	/prefix	/prefix/	/prefix/extra	/other
Router Mounted At	/prefix	/prefix	/prefix	/prefix	/prefix
Match?	N/A	Yes	Yes	Yes	No
Response Sent	N/A	Hello from router	Hello from router	No response	No response

Key Moments - 2 Insights

Why does the router handle '/prefix' and '/prefix/' but not '/prefix/extra'?

Because the router is mounted at '/prefix' and only matches requests starting with that path. However, the router has a route defined only for '/', so '/prefix/extra' matches the router but does not match any route inside it, so no handler is called (see execution_table rows 1, 2, and 3).

What happens if a request URL does not start with the prefix?

The router does not match the request, so no handler inside the router runs and no response is sent from it (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution table, what response is sent when the request URL is '/prefix/'?

A "Hello from router"

B "No response from router"

C "404 Not Found"

D "Error"

💡 Hint

Check execution_table row 2 under 'Response Sent'

At which request URL does the router NOT handle the request?

A /prefix

B /prefix/extra

C /other

D /prefix/

💡 Hint

Look at execution_table rows where 'Match?' is 'No'

If the router was mounted at '/api', which request URL would it handle?

A /prefix

B /api

C /other

D /prefix/extra

💡 Hint

Router matches requests starting with its mount path, see variable_tracker 'Router Mounted At'

Concept Snapshot

Express virtual path prefixes let you mount routers at specific paths.
Use app.use('/prefix', router) to route requests starting with '/prefix' to that router.
Router paths are relative to the prefix.
Requests not matching the prefix skip the router.
This helps organize routes under common URL segments.

Full Transcript

In Express, you can create a router and attach it to your main app at a specific path prefix. When a request comes in, Express checks if the URL starts with that prefix. If it does, the router handles the request using its defined routes. For example, mounting a router at '/prefix' means requests to '/prefix' or '/prefix/' trigger the router's handlers. Requests to '/prefix/extra' or other paths do not match the router's root route and so are not handled by it. This lets you group routes logically under a common path segment, making your app easier to manage.