The Top Express.js Interview Questions and Answers for 2023

Express.js has become one of the most popular backends web application frameworks for Node.js. With its minimalist approach, robust routing, and middleware capabilities, Express makes it easy to build fast and scalable server-side applications.

As Express continues to grow in popularity knowledge of the framework has become a highly sought-after skillset for full-stack and backend roles. To help you prepare for Express-related questions in upcoming interviews I’ve compiled some of the most common and important Express interview questions and answers below.

What is Express.js?

Expressjs is a web application framework for Nodejs that allows you to set up middleware and routing to quickly build web and mobile apps. It provides a thin layer on top of Node.js with web application features like routing, middleware, template engines, etc.

Some key features of Express include:

• Minimalist web framework for Node.js
• Robust routing system
• Integrated middleware support
• Template engines like Pug, Mustache, EJS
• Content negotiation
• Uses HTTP utility methods and middleware
• Easy to get started with Express Generator tool

Express makes it very easy to build single-page, multi-page, and hybrid web applications. It is unopinionated, meaning you can structure your app however you want without any restrictions. Over 65% of Node.js users are estimated to be using Express in production.

What are the key features of Express.js?

• Routing – Maps HTTP requests to handler functions based on the request URL and method. Supports parameterized routing as well.

• Middleware – Express apps can use middleware functions that process requests before they reach route handlers. Examples: logging, authentication, etc.

• Template engines – Express supports template engines like Pug, Mustache, EJS for server-side rendering of dynamic content.

• Content negotiation – Express can return different representations of resource data based on client preferences.

• Error handling – Express has a built-in error handler so you don’t have to write try/catch blocks everywhere.

• Serving static files – Express can serve static files like images, CSS, JS files, etc.

• CORS support – Express supports Cross-Origin Resource Sharing out of the box.

• HTTP helpers – Helper methods like res.redirect() and res.sendStatus() make sending responses easier.

How do you create an Express app?

Creating an Express app is very straightforward. Here are the basic steps:

1. Install Express using npm install express
2. Require the express module – const express = require('express')
3. Create the Express app – const app = express()
4. Define routes and mount them on the app
5. Configure middleware as needed
6. Define a port and start the server – app.listen(3000)

The app object has methods like app.get(), app.post() etc. to define routes. You can also use app.use() to mount middleware functions.

How does routing work in Express.js?

Routing refers to mapping HTTP requests to handler functions that process them.

In Express, you define routes using methods like app.get, app.post, app.put, app.delete etc. The first argument is the URL path, and the second argument is the route handler function.

For example:

app.get('/', function (req, res) {  res.send('Home page')})app.post('/contact', function (req, res) {  // Handle contact form submit})

Route paths can use parameters as well:

app.get('/users/:userId', function (req, res) {  res.send(req.params.userId) })

Express router can be used to modularize routes into separate files.

What is Express middleware?

Middleware functions are functions that have access to the request and response objects. They are invoked sequentially in the order they are defined when a request lands on the server.

Some common use cases of middleware:

• Logging
• Authentication
• Form data parsing
• File uploads
• Rate limiting
• Compression

Middleware can perform tasks like:

• Execute code
• Make changes to request/response objects
• End response cycle
• Call next middleware in stack

Express middleware example:

app.use(function(req, res, next) {  console.log(req.method, req.url)  next()})

The next() function passes control to the next middleware.

How do you handle errors in Express.js?

When asynchronous code throws errors, Express comes with a built-in error handler that catches and processes them.

You define error-handling middleware like other middleware, except with 4 arguments instead of 3. It should be defined after other app.use() and routes:

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

For granular control, you can detect errors in routes and pass them to next():

app.get('/', function (req, res, next) {  fs.readFile('/file-does-not-exist', function (err, data) {    if (err) {      next(err)     } else {      res.send(data)    }  })})

How does Express integrate with template engines?

Express supports template engines like Pug, Mustache and EJS. They allow you to dynamically generate HTML on the server-side and send it as response.

To use a template engine with Express, you need to install its Node module first. Then set the engine and view folder in Express settings.

app.set('view engine', 'pug')app.set('views', path.join(__dirname, 'views'))

Now you can render templates inside routes by calling res.render():

app.get('/', function (req, res) {  res.render('index', {title: 'Home Page'})  })

The first argument is the template name, and second is data to render in the template.

What is the difference between app.route() and app.all() in Express?

• app.route() is used to create chainable route handlers for a route path. It reduces duplicate route names and code.

• app.all() matches all HTTP verbs on a path. It can handle all HTTP methods for a path – GET, POST, PUT, DELETE etc.

For example:

app.route('/users')  .get(handleGetUsers)  .post(handleCreateUser)  .put(handleUpdateUser)// vsapp.all('/users', function (req, res, next) {  // Runs for all HTTP verbs on /users path})

So app.route() is used to avoid duplicate route names while app.all() matches all HTTP methods on a path.

How do you structure an Express application?

There is no definitive answer, but here is a common and clean way to structure an Express app:

myapp/├── app.js├── routes/│   └── index.js│   └── users.js├── models/│   └── user.js├── views/│   └── index.pug│   └── layout.pug│   └── error.pug├── public/│   └── css│   └── img│   └── js└── package.json

• app.js – The main app file where you configure settings, middleware, routes and start the server.
• routes/ – Keep route handlers separated in module files based on functionality.
• models/ – Database model schemas and model functions.
• views/ – Template files for rendering HTML.
• public/ – Static assets like CSS, JS, images.
• package.json – NPM dependencies and scripts.

This structure allows clean separation of concerns and modularity.

How do you implement authentication in Express apps?

Some ways to implement authentication in Express:

• HTTP based authentication using middleware functions like basicAuth.
• OAuth based authentication using passport.js.
• JWT (JSON web token) based authentication – maintain stateless auth.
• Sessions to keep user logged in across requests.
• Storing hashed passwords in database for authentication.

A typical user authentication flow involves:

• User signs up by providing credentials.
• Credentials are validated and hashed password is stored in DB.
• User logs in by providing credentials which are validated against DB.
• After authentication, a token is issue