Routing

Routing refers to the definition of end points (URIs) to an application and how it responds to client requests.

A route is a combination of a URI, a HTTP request method (GET, POST, and so on), and one or more handlers for the endpoint. It takes the following structure app.METHOD(path, [callback...], callback), where app is an instance of express, METHOD is an HTTP request method, path is a path on the server, and callback is the function executed when the route is matched.

The following is an example of a very basic route.

var express = require('express')
var app = express()

// respond with "hello world" when a GET request is made to the homepage
app.get('/', function(req, res) {
  res.send('hello world')
})

Route methods

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

The following is an example of routes defined for the GET and the POST methods to the root of the app.

// GET method route
app.get('/', function (req, res) {
  res.send('GET request to the homepage')
})

// POST method route
app.post('/', function (req, res) {
  res.send('POST request to the homepage')
})

Express supports the following routing methods corresponding to HTTP methods: get, post, put, head, delete, options, trace, copy, lock, mkcol, move, purge, propfind, proppatch, unlock, report, mkactivity, checkout, merge, m-search, notify, subscribe, unsubscribe, patch, search, and connect.

There is a special routing method, app.all(), which is not derived from any HTTP method. It is used for loading middleware at a path for all request methods.

In the following example, the handler will be executed for requests to “/secret” whether using GET, POST, PUT, DELETE, or any other HTTP request method.

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 to. They can be strings, string patterns, or regular expressions.

Examples of route paths based on strings:

// with match request to the root
app.get('/', function (req, res) {
  res.send('root')
})

// will match requests to /about
app.get('/about', function (req, res) {
  res.send('about')
})

// will match request to /random.text
app.get('/random.text', function (req, res) {
  res.send('random.text')
})

Examples of route paths based on string patterns:

// will match acd and abcd
app.get('/ab?cd', function(req, res) {
  res.send('ab?cd')
})

// will match abcd, abbcd, abbbcd, and so on
app.get('/ab+cd', function(req, res) {
  res.send('ab+cd')
})

// will match abcd, abxcd, abRABDOMcd, ab123cd, and so on
app.get('/ab*cd', function(req, res) {
  res.send('ab*cd')
})

// will match /abe and /abcde
app.get('/ab(cd)?e', function(req, res) {
 res.send('ab(cd)?e')
})

Examples of route paths based on regular expressions:

// will match anything with an a in the route name:
app.get(/a/, function(req, res) {
  res.send('/a/')
})

// will match butterfly, dagonfly; but not butterflyman, dragonfly man, and so on
app.get(/.*fly$/, function(req, res) {
  res.send('/.*fly$/')
})

Route handlers

You can provide multiple callback functions that behave just like middleware to handle a request. The only exception is that these callbacks may invoke next('route') to bypass the remaining route callback(s). 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 come in the form of a function, an array of functions, or various combinations of both, as shown the following examples.

A route can be handled using a single callback function:

app.get('/example/a', function (req, res) {
  res.send('Hello from A!')
})

A route can be handled using a more than one callback function (make sure to specify the next object):

app.get('/example/b', function (req, res, next) {
  console.log('response will be sent by the next function ...')
  next()
}, function (req, res) {
  res.send('Hello from B!')
})

A route can be handled using an array of callback functions:

var cb0 = function (req, res, next) {
  console.log('CB0')
  next()
}

var cb1 = function (req, res, next) {
  console.log('CB1')
  next()
}

var cb2 = function (req, res) {
  res.send('Hello from C!')
}

app.get('/example/c', [cb0, cb1, cb2])

A route can be handled using a combination of array of functions and independent functions:

var cb0 = function (req, res, next) {
  console.log('CB0')
  next()
}

var cb1 = function (req, res, next) {
  console.log('CB1')
  next()
}

app.get('/example/d', [cb0, cb1], function (req, res, next) {
  console.log('response will be sent by the next function ...')
  next()
}, function (req, res) {
  res.send('Hello from D!')
})

Response methods

The methods on the response object (res) in the following table can send a response to the client and terminate the request response cycle. If none of them is called from a route handler, the client request will be left hanging.

app.route()

Chainable route handlers for a route path can be created using app.route(). Since the path is specified at a single location, it helps to create modular routes and reduce redundancy and typos. For more information on routes, see Router() documentation.

Here is an example of chained route handlers defined 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

The express.Router class can be used 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 in it, defines some routes, and mounts it on a path on 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 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);

The app will now be able to handle requests to /birds and /birds/about, along with calling the timeLog middleware specific to the route.