Q: What is Express.js?

A: Express.js is a minimalistic and flexible web application framework for Node.js. It provides a robust set of features for building web applications and APIs, such as routing, middleware support, and template engines. Express.js simplifies the process of creating server-side applications in JavaScript by providing a lightweight framework that handles the complexities of handling HTTP requests and responses.

Q: How do you install Express.js?

A: To install Express.js, you need to have Node.js and npm (Node Package Manager) installed. Once you have them set up, you can install Express.js by running the following command in your terminal:

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npm install express

This command installs Express.js and its dependencies locally in your project’s node_modules directory.

Q: What is middleware in Express.js?

A: Middleware in Express.js refers to a series of functions that are executed sequentially in the request-response cycle. Each middleware function has access to the request (req) and response (res) objects, as well as a next function to pass control to the next middleware in the chain. Middleware functions can perform various tasks such as modifying the request/response objects, logging, authentication, error handling, and more. Express.js allows you to define custom middleware functions and use built-in middleware or third-party middleware packages.

Q: How do you handle routes in Express.js?

A: Express.js provides a routing mechanism that allows you to define routes for different HTTP methods (GET, POST, PUT, DELETE, etc.) and URLs. You can define routes using the app object, which represents the Express application. Here’s an example of defining a route for the root URL (“/”):

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app.get(‘/’, (req, res) => {

res.send(‘Hello, Express!’);

});

In this example, the get method is used to handle an HTTP GET request to the root URL (“/”). When a user accesses that URL, the callback function is executed, and the response with the message “Hello, Express!” is sent back.

Q: How can you access route parameters in Express.js?

A: Route parameters in Express.js are specified in the URL pattern using a colon followed by the parameter name. For example, to define a route that accepts a user ID as a parameter, you can do the following:

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app.get(‘/users/:id’, (req, res) => {

const userId = req.params.id;

// Use the userId in your logic

res.send(‘User ID: ‘ + userId);

});

In this case, when a user accesses “/users/123”, the value “123” will be accessible as req.params.id. You can then use this parameter value in your route handler to perform specific operations based on the user ID.

Q: How can you handle form data in Express.js?

A: To handle form data in Express.js, you need to use a middleware called body-parser (which used to be part of Express.js but is now a separate package). First, you need to install it by running:

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npm install body-parser

Once installed, you can use it in your Express application as follows:

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const bodyParser = require(‘body-parser’);

app.use(bodyParser.urlencoded({ extended: false }));

app.post(‘/submit’, (req, res) => {

const formData = req.body;

// Process the form data

res.send(‘Form submitted successfully’);

});

In this example, the bodyParser.urlencoded middleware is used to parse URL-encoded form data. It adds a body property to the req object, which contains the parsed form data.

Q: How do you handle errors in Express.js?

A: Express.js provides error handling middleware that allows you to handle errors that occur during the request-response cycle. You can define error-handling middleware functions with four parameters (err, req, res, next), where err represents the error object.

Here’s an example of error handling middleware in Express.js:

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app.use((err, req, res, next) => {

// Handle the error

res.status(500).send(‘Internal Server Error’);

});

In this example, the error-handling middleware is defined using app.use(). Whenever an error occurs in any middleware or route handler, Express will pass the error to this middleware, and you can handle it accordingly. In this case, the middleware responds with a 500 status code and sends the message “Internal Server Error” to the client.

Q: How can you serve static files in Express.js?

A: Express.js provides a built-in middleware function called express.static to serve static files such as CSS, JavaScript, images, and more. You can specify a directory from which to serve the static files using the following code:

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app.use(express.static(‘public’));

In this example, the “public” directory will be served as the static file directory. You can place your static files in this directory, and Express will automatically serve them. For example, if you have a file named “styles.css” inside the “public” directory, you can access it in the browser at http://localhost:3000/styles.css.

Q: How do you set up a template engine in Express.js?

A: Express.js allows you to use template engines to generate dynamic HTML content. There are various template engines available for Express, such as EJS, Pug (formerly Jade), Handlebars, etc. To set up a template engine, you need to install the corresponding package and configure it in your Express application.

Here’s an example of setting up the EJS template engine:

Install the EJS package:

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npm install ejs

Configure Express to use EJS as the template engine:

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app.set(‘view engine’, ‘ejs’);

Create a “views” directory in your project’s root directory and place your EJS templates inside it.

Render an EJS template in a route handler:

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app.get(‘/’, (req, res) => {

res.render(‘index’, { title: ‘Express Example’ });

});

In this example, when a user accesses the root URL (“/”), Express will render the “index.ejs” template and pass it an object containing the “title” variable. The template engine will merge the data with the template and send the resulting HTML to the client.

Q: How can you handle authentication in Express.js?

A: Authentication in Express.js can be handled using various strategies and middleware libraries. One popular choice is the Passport.js middleware, which provides a flexible authentication framework. To use Passport.js, you need to install it and any desired authentication strategy packages.

Here’s a general outline of setting up Passport.js for local username/password authentication:

Install Passport.js and the required strategy packages:

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npm install passport passport-local

Configure Passport.js and set up the authentication strategy:

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const passport = require(‘passport’);

const LocalStrategy = require(‘passport-local’).Strategy;

// Configure the local strategy

passport.use(new LocalStrategy(

(username, password, done) => {

// Implement your authentication logic here

// Call done() with the appropriate arguments

}

));

// Initialize Passport.js middleware

app.use(passport.initialize());

app.use(passport.session());

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