Top Express.js Interview Questions and Answers

What is Express.js?

Express.js (often just called Express) is a fast, minimalist web application framework for Node.js. It's open-source, written in JavaScript, and is widely used for building web applications and APIs. Express is known for its flexibility, simplicity, and efficiency.

Key Features of Express.js

• Supports building various web applications (single-page, multi-page, and hybrid).
• Uses middleware to process requests.
• Defines routing tables to map URLs to handler functions.
• Supports templating engines for dynamic HTML rendering.
• High performance due to its minimalist design.
• Good database integration.
• Asynchronous, single-threaded architecture.

Express.js: Front-end or Back-end?

Express.js is a back-end framework. It runs on the Node.js platform and is used to create server-side applications that handle requests and generate responses, typically serving HTML and data to front-end frameworks or directly to web browsers. It's a core component of the MEAN stack (MongoDB, Express.js, AngularJS, Node.js).

Reasons to Use Express.js

Express.js simplifies building Node.js web applications. Its minimalist and flexible design makes it highly efficient and easy to learn. It allows for rapid prototyping and scales effectively.

Express.js vs. Node.js

Example Express.js Code (GET Request)

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

app.get('/', (req, res) => {
  res.send('Hello from Express!');
});

app.listen(3000, () => console.log('Server listening on port 3000'));
        

Handling POST Requests in Express.js

To handle POST requests, you need to use body-parsing middleware (like body-parser) to access the request body data.

const bodyParser = require('body-parser');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
        

Scaffolding in Express.js

Scaffolding is a tool to automatically generate the basic structure of a web application. This helps to quickly set up a project, saving time and effort. Tools like the Express application generator and Yeoman simplify this process.

Scaffolding in Other MVC Frameworks

Many other MVC frameworks (like Ruby on Rails, Django, Laravel, etc.) support scaffolding to automate project setup.

Express.js Route Handler Function Arguments

An Express.js route handler function typically takes these arguments:

• req (request object): Contains information about the incoming request.
• res (response object): Used to send the response to the client.
• next (optional): A function for passing control to the next middleware function.

Express.js vs. Django

Enabling Debugging in Express.js

Set the DEBUG environment variable: DEBUG=express:* node app.js (Linux/macOS) or set DEBUG=express:* && node app.js (Windows).

Enabling CORS (Cross-Origin Resource Sharing) in Express.js

Add middleware to handle CORS headers. The following example allows requests from any origin:

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
  next();
});
        

Error Handling in Express.js

Use error-handling middleware to catch and handle errors that occur during request processing. This allows for graceful handling of exceptions and prevents application crashes.

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});
        

Error Handling in Express.js

Effective error handling is crucial for creating robust applications. In Express.js versions 4.0 and later, dedicated error-handling middleware is used to catch and manage errors during request processing. This helps to prevent application crashes and provides a structured way to respond to errors gracefully.

1. Create an Express app.
2. Create error-handling middleware: This middleware function takes four arguments (err, req, res, next). It logs the error, sets the response status code, and sends an error response to the client.
3. (Optional) Install and use an error handler middleware package: Packages like errorhandler simplify error handling by providing additional logging and reporting features.

//Error Handling Middleware
app.use(function(err, req, res, next) {
  console.error(err.stack);
  res.status(err.status || 500).send('Something broke!');
});
        

Serving Static Files in Express.js

The express.static() middleware function serves static files (HTML, CSS, JavaScript, images, etc.) from a specified directory. This simplifies serving assets without needing to handle them individually.

app.use(express.static('public')); //Serves files from the 'public' directory
        

Middleware in Express.js

Middleware functions are invoked by Express.js to process requests before they reach their final handler. They allow modifying requests, sending responses, and calling the next middleware function in the chain.

Types of Middleware

• Application-level: Applied to all routes (using app.use()).
• Router-level: Applied to specific routes (using router.use()).
• Error-handling: Handles errors that occur during request processing.
• Built-in Middleware: Provided by Express.js (static(), json(), urlencoded()).
• Third-party Middleware: Middleware from other packages (e.g., body-parser, cookie-parser).

Using Third-Party Middleware (e.g., body-parser)

Third-party middleware extends Express.js's functionality. Install using npm install package_name and require it in your app (e.g., body-parser for handling POST requests).

const bodyParser = require('body-parser');
app.use(bodyParser.json()); // for parsing application/json
app.use(bodyParser.urlencoded({ extended: true })); // for parsing application/x-www-form-urlencoded
        

Multiple Middleware on a Route

Multiple middleware functions can be applied to a single route by passing them as an array to app.get() or other routing methods.

const middleware1 = (req, res, next) => { /* ... */ next(); };
const middleware2 = (req, res, next) => { /* ... */ next(); };

app.get('/route', [middleware1, middleware2], (req, res) => { /* ... */ });
        

Template Engines Supported by Express.js

Express supports various templating engines (like Pug, EJS, Handlebars). Choose an engine that best suits your project.

Rendering Plain HTML in Express.js

For simple HTML, use res.send(). For serving static files from a directory, use express.static() middleware. The res.render() method is used for templating engines.