Top Express.js Interview Questions to Ace Your Next Job Interview

Express.js is a popular web application framework for Node.js, designed for building web applications and APIs. It is known for its simplicity, flexibility, and scalability. If you’re preparing for a job interview that involves Express.js, this article will cover some of the most common Express.js interview questions to help you get ready.

1. What is Express.js and why is it used?

Express.js is a lightweight, fast, and unopinionated web application framework for Node.js. It is used to build web applications, APIs, and other server-side applications with ease. Express.js simplifies the process of creating server-side applications by providing a robust set of features and middleware for handling HTTP requests and responses, routing, and other essential tasks.

2. How do you create a basic Express.js application?

To create a basic Express.js application, follow these steps:

1. Install Express.js using npm:

   npm install express

2. Create a new JavaScript file, e.g.,

   app.js

3. Import the Express.js module and create an instance of the Express.js application:

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

4. Define a route for handling HTTP requests:

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

5. Start the server on a specific port:

const port = 3000;
app.listen(port, () => {
  console.log(`Server running on port ${port}`);
});

3. What is middleware in Express.js?

Middleware is a function that has access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. Middleware functions can execute any code, make changes to the request and response objects, or end the request-response cycle. If a middleware function does not end the request-response cycle, it must call the next middleware function to pass control to the next middleware, otherwise, the request will be left hanging.

4. How do you use middleware in Express.js?

To use middleware in Express.js, you can use the

app.use()

function. Here’s an example of a simple middleware function that logs the request method and URL:

const loggerMiddleware = (req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
};

app.use(loggerMiddleware);

This middleware function will be executed for every incoming request, logging the request method and URL before passing control to the next middleware or route handler.

5. How do you handle errors in Express.js?

Error handling in Express.js is done using error-handling middleware functions. These functions have four arguments instead of three:

(err, req, res, next)

. To define an error-handling middleware, you can use the following syntax:

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

This error-handling middleware will log the error stack and send a 500 Internal Server Error response to the client.