Routing

Routing refers to how an application’s endpoints (URIs) respond to client requests.

You define routing using methods of the Express app object that correspond to HTTP methods

for example, app.get() to handle GET requests and app.post to handle POST requests. You can also use app.all() to handle all HTTP methods and app.use() to specify middleware as the callback function (See Using middleware for details).

These routing methods specify a callback function called when the application receives a request to the specified route (endpoint) and HTTP method. In other words, the application “listens” for requests that match the specified route(s) and method(s), and when it detects a match, it calls the specified callback function.

In fact, the routing methods can have more than one callback function as arguments. With multiple callback functions, it is important to call provide next as an argument to the callback function and then call next() within the body of the function to hand off control to the next callback.

Route methods

A route method is derived from one of the HTTP methods, and is attached to an instance of the express class.

There is a special routing method, app.all(), used to load middleware functions at a path for all HTTP request methods.

app.all('/secret', function (req, res, next) {
  console.log('Accessing the secret section ...')
  next() // pass control to the next handler
})

Route paths

Route paths, in combination with a request method, define the endpoints at which requests can be made. Route paths can be strings, string patterns, or regular expressions.

Route parameters

Route parameters are named URL segments that are used to capture the values specified at their position in the URL. The captured values are populated in the req.params object, with the name of the route parameter specified in the path as their respective keys.

Route path: /users/:userId/books/:bookId
Request URL: http://localhost:3000/users/34/books/8989
req.params: { "userId": "34", "bookId": "8989" }

Route handlers

You can provide multiple callback functions that behave like middleware to handle a request. The only exception is that these callbacks might invoke next('route') to bypass the remaining route callbacks. You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there’s no reason to proceed with the current route.

Route handlers can be in the form of a function, an array of functions, or combinations of both, as shown in the following examples.

express.Router

Use the express.Router class to create modular, mountable route handlers. A Router instance is a complete middleware and routing system; for this reason, it is often referred to as a “mini-app”.

The following example creates a router as a module, loads a middleware function in it, defines some routes, and mounts the router module on a path in the main app.

var express = require('express')
var router = express.Router()

// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
  console.log('Time: ', Date.now())
  next()
})
// define the home page route
router.get('/', function (req, res) {
  res.send('Birds home page')
})
// define the about route
router.get('/about', function (req, res) {
  res.send('About birds')
})

module.exports = router

Then, load the router module in the app:

var birds = require('./birds')

// ...

app.use('/birds', birds)

app.route()

You can create chainable route handlers for a route path by using app.route(). Because the path is specified at a single location, creating modular routes is helpful, as is reducing redundancy and typos. For more information about routes, see: Router() documentation.

Here is an example of chained route handlers that are defined by using app.route().

app.route('/book')
  .get(function (req, res) {
    res.send('Get a random book')
  })
  .post(function (req, res) {
    res.send('Add a book')
  })
  .put(function (req, res) {
    res.send('Update the book')
  })

express.Router

Use the express.Router class to create modular, mountable route handlers. A Router instance is a complete middleware and routing system; for this reason, it is often referred to as a “mini-app”.

The following example creates a router as a module, loads a middleware function in it, defines some routes, and mounts the router module on a path in the main app.

Create a router file named birds.js in the app directory, with the following content:

var express = require('express')
var router = express.Router()

// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
  console.log('Time: ', Date.now())
  next()
})
// define the home page route
router.get('/', function (req, res) {
  res.send('Birds home page')
})
// define the about route
router.get('/about', function (req, res) {
  res.send('About birds')
})

module.exports = router