Route prefixing in Express - Step-by-Step Execution

Concept Flow - Route prefixing

Create Express app

↓

Create Router

↓

Define routes on Router

↓

Use Router with prefix

↓

App listens for requests

↓

Request comes in

↓

Check prefix match

Yes↓

Route handler runs

↓

Send response

↓

End

This flow shows how an Express app uses a router with a prefix to handle requests starting with that prefix.

Execution Sample

Express

const express = require('express');
const app = express();
const router = express.Router();
router.get('/hello', (req, res) => res.send('Hi'));
app.use('/api', router);
app.listen(3000);

This code creates a router with a '/hello' route, then prefixes it with '/api' in the main app.

Execution Table

Step	Action	Request URL	Prefix Match?	Route Matched	Response Sent
1	App starts listening	-	-	-	-
2	Request received	/api/hello	Yes	/hello	'Hi'
3	Request received	/hello	No	No route	404 Not Found
4	Request received	/api/goodbye	Yes	No route	404 Not Found
5	Request received	/api/hello/world	Yes	No route	404 Not Found

💡 Requests stop processing when no matching route is found or response is sent.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
request.url	-	/api/hello	/hello	/api/goodbye	/api/hello/world
prefix '/api'	set	matched	not matched	matched	matched
route matched	none	/hello	none	none	none
response	none	'Hi'	404	404	404

Key Moments - 2 Insights

Why does '/hello' not match when prefix '/api' is used?

Because the router only handles routes starting with '/api'. The request '/hello' lacks this prefix, so it does not match any route in the router (see execution_table row 3).

What happens if the request URL has the prefix but no matching route?

The router checks the prefix and matches it, but since no route matches the rest of the path, it returns 404 Not Found (see execution_table rows 4 and 5).

Visual Quiz - 3 Questions

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

A 404 Not Found

B 'Hi'

C No response

D Error

💡 Hint

Check row 2 in the execution_table under 'Response Sent'

At which step does the prefix '/api' not match the request URL?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Prefix Match?' column in execution_table

If we change the prefix to '/v1' instead of '/api', what happens to the response for request '/api/hello'?

A Response is 404 Not Found

B Response is an error

C Response is 'Hi'

D Response is empty

💡 Hint

Changing prefix means '/api/hello' no longer matches the router prefix, see variable_tracker for prefix matching

Concept Snapshot

Route Prefixing in Express:
- Create a router with routes (e.g., router.get('/hello'))
- Use app.use('/prefix', router) to add a prefix
- Requests must start with prefix to reach router routes
- Router matches routes after prefix
- Non-matching routes or prefixes result in 404
- Helps organize routes under common path

Full Transcript

Route prefixing in Express means you create a router with some routes, then tell the main app to use that router but only for URLs starting with a certain prefix. When a request comes in, Express checks if the URL starts with the prefix. If yes, it removes the prefix and tries to match the rest of the path to the router's routes. If a route matches, its handler runs and sends a response. If no route matches, Express sends a 404 Not Found. If the prefix does not match, the router is skipped. This helps organize routes under a common path segment, like '/api'.