The stuff that makes up Express

A good thing about Express is that there are only three core components to it, which makes it relatively easy to know a lot about Express, if not master it entirely. In this section, I will give a brief introduction about each of the core Express components, so that you are not left disoriented when you come across them in the coming chapters.

The application object

The application object is an instance of Express, conventionally represented by the variable named app. This is the main object of your Express app and the bulk of the functionality is built on it.

This is how you create an instance of the Express module:

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

The following is a brief description of all the properties and methods available on the application object:

Property/Method	Description
app.set(name, value)	Sets app- specific properties
app.get(name)	Retrieves value set by app.set()
app.enable(name)	Enables a setting in the app
app.disable(name)	Disables a setting in the app
app.enabled(name)	Checks if a setting is enabled
app.disabled(name)	Checks if a setting is disabled
app.configure([env], callback)	Sets app settings conditionally based on the development environment
app.use([path], function)	Loads a middleware in the app
app.engine(ext, callback)	Registers a template engine for the app
app.param([name], callback)	Adds logic to route parameters
app.VERB(path, [callback...], callback)	Defines routes and handlers based on HTTP verbs
app.all(path, [callback...], callback)	Defines routes and handlers for all HTTP verbs
app.locals	The object to store variables accessible from any view
app.render(view, [options], callback)	Renders view from the app
app.routes	A list of routes defined in the app
app.listen()	Binds and listen for connections

The request object

The HTTP request object is created when a client makes a request to the Express app. The object is conventionally represented by a variable named req, which contains a number of properties and methods related to the current request.

The following table lists all the properties and methods of the req object and provides a brief description of them:

Property/Method	Description
req.params	Holds the values of named routes parameters
req.params(name)	Returns the value of a parameter from named routes or GET params or POST params
req.query	Holds the values of a GET form submission
req.body	Holds the values of a POST form submission
req.files	Holds the files uploaded via a form
req.route	Provides details about the current matched route
req.cookies	Cookie values
req.signedCookies	Signed cookie values
req.get(header)	Gets the request HTTP header
req.accepts(types)	Checks if the client accepts the media types
req.accepted	A list of accepted media types by the client
req.is(type)	Checks if the incoming request is of the particular media type
req.ip	The IP address of the client
req.ips	The IP address of the client, along with that of the proxies it is connected through
req.path	The request path
req.host	Hostname from the HTTP header
req.fresh	Checks if the request is still fresh
req.stale	Checks if the request is stale
req.xhr	Checks if the request came via an AJAX request
req.protocol	The protocol used for making the request
req.secure	Checks if it is a secure connection
req.subdomains	Subdomains of the host domain name
req.url	The request path, along with any query parameters
req.originalUrl	Used as a backup for req.url
req.acceptedLanguages	A list of accepted languages by the client
req.acceptsLanguage(langauge)	Checks if the client accepts the language
req.acceptedCharsets	A list of accepted charsets by the client
req.acceptsCharsets(charset)	Checks if the client accepts the charset

The response object

The response object is created along with the request object, and is conventionally represented by a variable named res. While it may sound a little strange that both of them should be created together, it is a necessity to give all the middlewares a chance to work on the request and the response object, before passing the control to the next middleware.

The following is a table of properties and methods on the response object:

Property/Method	Description
res.status(code)	Sets the HTTP response code
res.set(field, [value])	Sets response HTTP headers
res.get(header)	Gets the response HTTP header
res.cookie(name, value, [options])	Sets cookie on the client
res.clearCookie(name, [options])	Deletes cookie on the client
res.redirect([status], url)	Redirects the client to a URL, with an optional HTTP status code
res.location	The location value of the response HTTP header
res.charset	The charset value of the response HTTP header
res.send([body|status], [body])	Sends an HTTP response object, with an optional HTTP response code
res.json([status|body], [body])	Sends a JSON object for HTTP response, along with an optional HTTP response code
res.jsonp([status|body], [body])	Sends a JSON object for HTTP response with JSONP support, along with an optional HTTP response code
res.type(type)	Sets the media type HTTP response header
res.format(object)	Sends a response conditionally, based on the request HTTP Accept header
res.attachment([filename])	Sets response HTTP header Content-Disposition to attachment
res.sendfile(path, [options], [callback]])	Sends a file to the client
res.download(path, [filename], [callback])	Prompts the client to download a file
res.links(links)	Sets the HTTP Links header
res.locals	The object to store variables specific to the view rendering a request
res.render(view, [locals], callback)	Renders a view