express js

express()

Creates an Express application. The express() function is a top-level function exported by the express module.

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

Application

The app object conventially denotes the Express application. Create it by calling the top-level express() function exported by the Express module:

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

app.get('/', function(req, res){
  res.send('hello world');
});

app.listen(3000);

The app object has a methods for

• Routing HTTP requests; see for example, app.METHOD and app.param.
• Configuring middleware; see app.route.
• Rendering HTML views; see app.render.
• Registering a template engine; see app.engine.

It also has settings (properties) that affect how the application behaves; for more information, see Application settings.

Properties

app.locals

The app.locals object is a JavaScript object, and its properties are local variables within the application.

app.locals.title
// => 'My App'

app.locals.email
// => 'me@myapp.com'

Once set, the value of app.locals properties persist throughout the life of the application, in contrast with res.locals properties that are valid only for the lifetime of the request.
You can accesss local variables in templates rendered within the application. This is useful for providing helper functions to templates, as well as app-level data. Note, however, that you cannot access local variables in middleware.

app.locals.title = 'My App';
app.locals.strftime = require('strftime');
app.locals.email = 'me@myapp.com';

app.mountpath

The app.mountpath property is the path pattern(s) on which a sub app was mounted.

A sub app is an instance of express which may be used for handling the request to a route.

var express = required('express');

var app = express(); // the main app
var admin = express(); // the sub app

admin.get('/', function (req, res) {
  console.log(admin.mountpath); // /admin
  res.send('Admin Homepage');
})

app.use('/admin', admin); // mount the sub app

It is similar to the baseUrl property of the req object, except req.baseUrl returns the matched URL path, instead of the matched pattern(s).
If a sub-app is mounted on multiple path patterns, app.mountpath returns the list of patterns it is mounted on, as shown in the following example.

var admin = express();

admin.get('/', function (req, res) {
  console.log(admin.mountpath); // [ '/adm*n', '/manager' ]
  res.send('Admin Homepage');
})

var secret = express();
secret.get('/', function (req, res) {
  console.log(secret.mountpath); // /secr*t
  res.send('Admin Secret');
});

admin.use('/secr*t', secret); // load the 'secret' router on '/secr*t', on the 'admin' sub app
app.use(['/adm*n', '/manager'], admin); // load the 'admin' router on '/adm*n' and '/manager', on the parent app

Events

app.on('mount', callback(parent))

The mount event is fired on a sub-app, when it is mounted on a parent app. The parent app is passed to the callback function.

var admin = express();

admin.on('mount', function (parent) {
  console.log('Admin Mounted');
  console.log(parent); // refers to the parent app
});

admin.get('/', function (req, res) {
  res.send('Admin Homepage');
});

app.use('/admin', admin);

Methods

app.all(path, callback [, callback ...])

This method is like the standard app.METHOD() methods, except it matches all HTTP verbs.
It’s useful for mapping “global” logic for specific path prefixes or arbitrary matches. For example, if you put the following at the top of all other route definitions, it requires that all routes from that point on require authentication, and automatically load a user. Keep in mind that these callbacks do not have to act as end-points: loadUser can perform a task, then call next() to continue matching subsequent routes.

app.all('*', requireAuthentication, loadUser);

Or the equivalent:

app.all('*', requireAuthentication)
app.all('*', loadUser);

Another example is white-listed “global” functionality. The example is much like before, however it only restricts paths that start with “/api”:

app.all('/api/*', requireAuthentication);

app.delete(path, callback [, callback ...])

Routes HTTP DELETE requests to the specified path with the specified callback functions. For more information, see the routing guide.
You can provide multiple callback functions that behave just like middleware, except these callbacks can 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.

app.delete('/', function (req, res) {
  res.send('DELETE request to homepage');
});

app.disable(name)

Sets the Boolean setting name to false, where name is one of the properties from the app settings table. Calling app.set('foo', false) for a Boolean property is the same as calling app.disable('foo').
For example:

app.disable('trust proxy');
app.get('trust proxy');
// => false

app.disabled(name)

Returns true if the Boolean setting name is disabled (false), where name is one of the properties from the app settings table.

app.disabled('trust proxy');
// => true

app.enable('trust proxy');
app.disabled('trust proxy');
// => false

app.enable(name)

Sets the Boolean setting name to true, where name is one of the properties from the app settings table. Calling app.set('foo', true) for a Boolean property is the same as calling app.enable('foo').

app.enable('trust proxy');
app.get('trust proxy');
// => true

app.enabled(name)

Returns true if the setting name is enabled (true), where name is one of the properties from the app settings table.

app.enabled('trust proxy');
// => false

app.enable('trust proxy');
app.enabled('trust proxy');
// => true

app.engine(ext, callback)

Registers the given template engine callback as ext.
By default, Express will require() the engine based on the file extension. For example, if you try to render a “foo.jade” file, Express invokes the following internally, and caches the require() on subsequent calls to increase performance.

app.engine('jade', require('jade').__express);

Use this method for engines that do not provide .__express out of the box, or if you wish to “map” a different extension to the template engine.
For example, to map the EJS template engine to “.html” files:

app.engine('html', require('ejs').renderFile);

In this case, EJS provides a .renderFile() method with the same signature that Express expects: (path, options, callback), though note that it aliases this method as ejs.__express internally so if you’re using “.ejs” extensions you don’t need to do anything.
Some template engines do not follow this convention. The consolidate.js library maps Node template engines to follow this convention, so they work seemlessly with Express.

var engines = require('consolidate');
app.engine('haml', engines.haml);
app.engine('html', engines.hogan);

app.get(name)

Returns the value of name app setting, where name is one of strings in the app settings table. For example:

app.get('title');
// => undefined

app.set('title', 'My Site');
app.get('title');
// => "My Site"

app.get(path, callback [, callback ...])

Routes HTTP GET requests to the specified path with the specified callback functions. For more information, see the routing guide.
You can provide multiple callback functions that behave just like middleware, except these callbacks can 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.

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

app.listen(port, [hostname], [backlog], [callback])

Binds and listens for connections on the specified host and port. This method is identical to Node’s http.Server.listen().

var express = require('express');
var app = express();
app.listen(3000);

The app returned by express() is in fact a JavaScript Function, designed to be passed to Node’s HTTP servers as a callback to handle requests. This makes it easy to provide both HTTP and HTTPS versions of your app with the same code base, as the app does not inherit from these (it is simply a callback):

var express = require('express');
var https = require('https');
var http = require('http');
var app = express();

http.createServer(app).listen(80);
https.createServer(options, app).listen(443);

The app.listen() method is a convenience method for the following (for HTTP only):

app.listen = function() {
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};

app.METHOD(path, callback [, callback ...])

Routes an HTTP request, where METHOD is the HTTP method of the request, such as GET, PUT, POST, and so on, in lowercase. Thus, the actual methods are app.get(), app.post(), app.put(), and so on. See below for the complete list.
For more information, see the routing guide.
Express supports the following routing methods corresponding to the HTTP methods of the same names:

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