Detect the matched router prefix
A router serves multiple mounted prefixes and needs to identify which prefix matched the current request.
Express.js · Pipe
req.baseUrl
req.baseUrl is the URL path where the current Express router was mounted. It reports the concrete path matched for the request, not the pattern used to mount the router.
How it works
req.baseUrl is a request property containing the path where the current router was mounted. It reports the concrete matched path, even when the router was mounted using a pattern or several patterns; req.originalUrl combines this value with req.url.
Syntax
req.baseUrlParameters
| Name | Type | Description |
|---|---|---|
req | Express request object | The Express request object whose mounted-router path is being read. |
Returns
String: the URL path on which a router instance was mounted.
Key points
- The property identifies the URL prefix where the active router was mounted.
- It parallels
app.mountpath, but it is read from the request and describes the router involved in that request. - Unlike the request property,
app.mountpathexposes matched path pattern definitions. - For a request under
/greet, the router prefix is exposed as/greet. - A different mounted prefix is reflected directly when the request uses
/hello. - Pattern-based mounting still yields the path text that matched the incoming request.
- To relate the mounted prefix to the full original URL, use
req.originalUrl, which combines the base and router URL.
Common mistakes
- When mounting with path patterns, do not expect the pattern definition; the property contains the actual matched path string.
- Use the router's request context when comparing the mounted prefix with the complete incoming URL; the related original URL is available in both middleware and router objects.
Example: Read a router's mounted path
const express = require('express');
const app = express();
const router = express.Router();
router.get('/jp', (req, res) => {
res.json({ baseUrl: req.baseUrl, originalUrl: req.originalUrl });
});
app.use('/greet', router);
app.listen(3000);Run the app and request /greet/jp to see /greet as the mounted router path and /greet/jp as the original URL.
When to use it
Separate mount path from route path
Middleware needs to distinguish a router's mounted path from the remaining request URL.
Log routing context
Diagnostics need to compare the mounted prefix with the full original request URL.
Interview questions
QWhat does req.baseUrl represent?
It is the URL path on which the current router instance was mounted. For a request to /greet/jp handled by a router mounted at /greet, its value is /greet.
Follow-up: How does it differ from app.mountpath? req.baseUrl contains the matched string for the request, whereas app.mountpath returns the matched path pattern or patterns.
QWhat happens when a router is mounted with a path pattern?
A router mounted with a pattern can match different concrete paths. req.baseUrl reports the concrete matched string rather than the pattern definition.
Follow-up: How can the complete original request URL be reconstructed? The documentation describes req.originalUrl as a combination of req.baseUrl and req.url.
QHow does req.baseUrl relate to req.originalUrl?
req.baseUrl identifies the router's mounted prefix. req.originalUrl is the related complete URL value formed from req.baseUrl and req.url.
Follow-up: Which property can be rewritten for internal routing purposes? req.url can be freely rewritten, while req.originalUrl retains the original request URL.
Related
req.originalUrlreq.urlapp.mountpathreq.route