Working with Route Parameters
1. Defining URL Parameters
Path Example URL Captures /users/:id/users/42 id="42" /u/:id([0-9]+)/u/42 (numeric only) id="42" /files/:name.:ext/files/photo.jpg name, ext /posts/:slug-:id/posts/hello-42 slug, id
2. Accessing Parameters
Example: Reading params
app. get ( "/users/:id" , ( req , res ) => { const id = req.params.id; // string const numericId = Number (id); // coerce res. json ({ id: numericId }); });
Property Type req.paramsObject (always strings) req.params.idString
3. Using Multiple Parameters
Example: Nested resources
app. get ( "/categories/:cat/products/:id" , ( req , res ) => { const { cat , id } = req.params; res. json ({ category: cat, productId: id }); });
Pattern Result /:a/:bTwo separate path segments /:a-:bSame segment, hyphen separator
4. Validating Parameters
Example: Inline guard + express-validator
import { param, validationResult } from "express-validator" ; app. get ( "/users/:id" , param ( "id" ). isUUID (). withMessage ( "Invalid UUID" ), ( req , res ) => { const errors = validationResult (req); if ( ! errors. isEmpty ()) return res. status ( 400 ). json ({ errors: errors. array () }); res. json ({ id: req.params.id }); } );
Strategy Library Regex in path Built-in (:id(\\d+)) Validator middleware express-validator, zod Custom guard Plain function before handler
5. Making Parameters Optional
Example: Optional segment
// Express 4 app. get ( "/users/:id?" , ( req , res ) => { if (req.params.id) return res. json ( getUser (req.params.id)); res. json ( listUsers ()); }); // Express 5 — optional groups via {} app. get ( "/users{/:id}" , handler);
Syntax Express 4 Express 5 Optional :id?{:id}Wildcard *{*splat}
6. Using param() Middleware
Example: Centralized lookup
app. param ( "userId" , async ( req , res , next , id ) => { const user = await db.users. findById (id); if ( ! user) return res. sendStatus ( 404 ); req.user = user; next (); }); app. get ( "/users/:userId" , ( req , res ) => res. json (req.user)); app. put ( "/users/:userId" , ( req , res ) => updateUser (req.user, req.body));
Behavior Detail One per request Cached even if param appears in multiple routes Per-router Use router.param() for scoped lookups
7. Pre-processing Parameters
Example: Coerce + normalize
app. param ( "id" , ( req , res , next , raw ) => { const id = Number (raw); if ( ! Number. isInteger (id) || id <= 0 ) return res. status ( 400 ). json ({ error: "Invalid id" }); req.id = id; // attach typed value next (); });
8. Converting Parameter Types
Target Conversion Integer parseInt(p, 10) or Number(p)Float parseFloat(p)Boolean p === "true"Date new Date(p) + isNaN() checkUUID Validate against regex Array p.split(",")
9. Handling Invalid Parameters
Example: 400 vs 404
app. get ( "/users/:id" , async ( req , res ) => { const id = Number (req.params.id); if ( ! Number. isInteger (id)) return res. status ( 400 ). json ({ error: "id must be integer" }); const user = await db.users. findById (id); if ( ! user) return res. status ( 404 ). json ({ error: "User not found" }); res. json (user); });
Status When 400 Bad Request Param fails validation (wrong type/format) 404 Not Found Valid format but resource missing 422 Unprocessable Semantic validation failure
10. Using Wildcard Parameters
Example: Catch-all
// Express 4 app. get ( "/files/*" , ( req , res ) => { const path = req.params[ 0 ]; // anything after /files/ res. sendFile (path, { root: "uploads" }); }); // Express 5 named wildcard app. get ( "/files/{*splat}" , ( req , res ) => { const segments = req.params.splat; // array res. json ({ segments }); });
Syntax Captured As * (Express 4)req.params[0] (string){*splat} (Express 5)req.params.splat (array){*splat}?Optional wildcard
Warning: Always sanitize wildcard params used in file paths to prevent directory traversal (../).