Node.js – Express Framework

Node.js – Express Framework

Express Overview

Express is a minimal and flexible Node.js web application framework that provides a robust set of features to develop web and mobile applications. It facilitates the rapid development of Node based Web applications. Following are some of the core features of Express framework −

  • Allows to set up middlewares to respond to HTTP Requests.
  • Defines a routing table which is used to perform different actions based on HTTP Method and URL.
  • Allows to dynamically render HTML Pages based on passing arguments to templates.

    Installing Express

    Firstly, install the Express framework globally using NPM so that it can be used to create a web application using node terminal.

    $ npm install express --save

    The above command saves the installation locally in the node_modules directory and creates a directory express inside node_modules. You should install the following important modules along with express −

  • body-parser − This is a node.js middleware for handling JSON, Raw, Text and URL encoded form data.
  • cookie-parser − Parse Cookie header and populate req.cookies with an object keyed by the cookie names.
  • multer − This is a node.js middleware for handling multipart/form-data.
    $ npm install body-parser --save
    $ npm install cookie-parser --save
    $ npm install multer --save

    Hello world Example

    Following is a very basic Express app which starts a server and listens on port 3000 for connection. This app responds with Hello World! for requests to the homepage. For every other path, it will respond with a 404 Not Found.

    var express = require('express');
    var app = express();
    app.get('/', function(req, res){
    res.send('<h1>Connection meet</h1>');
    });
    var server = app.listen(3001, function(){
    console.log("server listning to port 3001");
    });

    Save the above code in a file named server.js and run it with the following command.

    $ node server.js

    You will see the following output −

    server listning to port 3001

    Open http://127.0.0.1:3001/ in any browser to see the following result.

    Request & Response

    Express application uses a callback function whose parameters are request and response objects.

    app.get('/', function (req, res) {
    // --
    })
  • Request Object − The request object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on.
  • Response Object − The response object represents the HTTP response that an Express app sends when it gets an HTTP request.
    You can print req and res objects which provide a lot of information related to HTTP request and response including cookies, sessions, URL, etc.

    Basic Routing

    We have seen a basic application which serves HTTP request for the homepage. Routing refers to determining how an application responds to a client request to a particular endpoint, which is a URI (or path) and a specific HTTP request method (GET, POST, and so on).
    We will extend our Hello World program to handle more types of HTTP requests.

    var express = require('express');
    var app = express();
    app.get('/', function (req, res) {
    res.send('<h1>Hello World</h1>');
    });
    app.post('/submit-data', function (req, res) {
    res.send('POST Request');
    });
    app.put('/update-data', function (req, res) {
    res.send('PUT Request');
    });
    app.delete('/delete-data', function (req, res) {
    res.send('DELETE Request');
    });
    var server = app.listen(3000, function () {
    console.log('Node server is running..');
    });

    Save the above code in a file named server.js and run it with the following command.

    $ node server.js

    You will see the following output −

    Example app listening at http://0.0.0.0:3000

    Now you can try different requests at http://127.0.0.1:3000 to see the output generated by server.js. Following are a few screens shots showing different responses for different URLs.
    Screen showing again http://127.0.0.1:3000/list_user
    Screen showing again http://127.0.0.1:3000/abcd
    Screen showing again http://127.0.0.1:3000/abcdefg

    Serving Static Files

    Express provides a built-in middleware express.static to serve static files, such as images, CSS, JavaScript, etc.
    You simply need to pass the name of the directory where you keep your static assets, to the express.static middleware to start serving the files directly. For example, if you keep your images, CSS, and JavaScript files in a directory named public, you can do this −

    app.use(express.static('public'));

    We will keep a few images in public/images sub-directory as follows −

    node_modules
    app.js
    public/
    public/images
    public/images/img.png
    public/images/img2.png

    Let's modify "Hello Word" app to add the functionality to handle static files.

    var path = require('path');
    var express = require('express');
    var app = express();
    app.use(express.static(path.join(__dirname, 'public')));
    app.get('/index.htm', function (req, res) {
    res.sendFile( __dirname + "/" + "index.htm" );
    })
    var server = app.listen(3000, function () {
    console.log('Node server is running.. to port 3000');
    });

    Save the above code in a file named app.js and run it with the following command.

    $ node app.js

    Now open http://127.0.0.1:3000/images/img.png in any browser and see observe following result.

    GET Method

    Here is a simple example which passes two values using HTML FORM GET method. We are going to use process_get router inside server.js to handle this input.

    First Name:  <br />
    Last Name:

    Let's save above code in index.htm and modify app.js to handle home page requests as well as the input sent by the HTML form.

    var express = require('express');
    var app = express();
    var path = require('path');
    app.use(express.static(path.join(__dirname, 'public')));
    app.get('/index.htm', function (req, res) {
    res.sendFile( __dirname + "/" + "index.html" );
    })
    app.get('/process_get', function (req, res) {
    // Prepare output in JSON format
    response = {
    first_name:req.query.first_name,
    last_name:req.query.last_name
    };
    console.log(response);
    res.end(JSON.stringify(response));
    })

    Accessing the HTML document using http://127.0.0.1:3000/index.htm will generate the following form −

    First Name:
    Last Name:
    First Name:
    Last Name:

    Now you can enter the First and Last Name and then click submit button to see the result and it should return the following result −
    ```
    { first_name: 'Prutor', last_name: 'Ai' }
    ```
    ## POST Method
    Here is a simple example which passes two values using HTML FORM POST method. We are going to use process_get router inside server.js to handle this input.
    ```
    First Name:
    Last Name:

    ```
    Let's save the above code in index.htm and modify server.js to handle home page requests as well as the input sent by the HTML form.
    ```
    var express = require('express');
    var app = express();
    var bodyParser = require('body-parser');
    var path = require('path');
    // Create application/x-www-form-urlencoded parser
    var urlencodedParser = bodyParser.urlencoded({ extended: false })
    app.use(express.static(path.join(__dirname, 'public')));
    app.get('/index.htm', function (req, res) {
    res.sendFile( __dirname + "/" + "index.htm" );
    })
    app.post('/process_post', urlencodedParser, function (req, res) {
    // Prepare output in JSON format
    response = {
    first_name:req.body.first_name,
    last_name:req.body.last_name
    };
    console.log(response);
    res.end(JSON.stringify(response));
    })
    var server = app.listen(3000,"127.0.0.1",function () {
    var host = server.address().address
    var port = server.address().port
    console.log("Example app listening at http://%s:%s", host, port)
    })
    ```
    Accessing the HTML document using http://127.0.0.1:3000/index.htm will generate the following form −
    ```
    First Name:
    Last Name:
    ```

    First Name:
    Last Name:

    Now you can enter the First and Last Name and then click the submit button to see the following result −
    ```
    { first_name: 'Prutor', last_name: 'Ai' }
    ```
    ## File Upload
    The following HTML code creates a file uploader form. This form has method attribute set to POST and enctype attribute is set to multipart/form-data
    ```

    Basic File Uploader

    Select a file to upload:

    ```
    Let's save above code in index.htm and modify server.js to handle home page requests as well as file upload.
    ```
    var express = require('express');
    var app = express();
    var fs = require("fs");
    var path = require('path')
    var bodyParser = require('body-parser');
    var multer = require('multer');
    app.use(express.static(path.join(__dirname, 'public')));
    app.use(bodyParser.urlencoded({ extended: false }));
    app.use(multer({ dest: '/tmp/'}));
    app.get('/index.htm', function (req, res) {
    res.sendFile( __dirname + "/" + "index.htm" );
    })
    app.post('/file_upload', function (req, res) {
    console.log(req.files.file.name);
    console.log(req.files.file.path);
    console.log(req.files.file.type);
    var file = __dirname + "/" + req.files.file.name;
    fs.readFile( req.files.file.path, function (err, data) {
    fs.writeFile(file, data, function (err) {
    if( err ) {
    console.log( err );
    } else {
    response = {
    message:'File uploaded successfully',
    filename:req.files.file.name
    };
    }
    console.log( response );
    res.end( JSON.stringify( response ) );
    });
    });
    })
    var server = app.listen(3000,"127.0.0.1", function () {
    var host = server.address().address
    var port = server.address().port
    console.log("Example app listening at http://%s:%s", host, port)
    })
    ```
    Accessing the HTML document using http://127.0.0.1:3000/index.htm will generate the following form −
    ```
    File Upload:
    Select a file to upload:
    NOTE: This is just dummy form and would not work, but it must work at your server.
    ```
    ## Cookies Management
    You can send cookies to a Node.js server which can handle the same using the following middleware option. Following is a simple example to print all the cookies sent by the client.
    ```
    var express = require('express')
    var cookieParser = require('cookie-parser')
    var app = express()
    app.use(cookieParser())
    app.get('/', function(req, res) {
    console.log("Cookies: ", req.cookies)
    })
    app.listen(3000,function(){
    console.log("server is listning to port 3000);
    })
    ```

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