Express js interview questions and answers ( 2024 Guide )

Do you dream of becoming a software engineer or more specifically a Node JS developer? Are you about to appear for an interview? Does the idea of ​​Express JS interview questions bother you? So take a moment and relax!

In this article, we will provide you with Express JS interview questions that are highly likely to be asked. Be sure to review them at least once before your interview.

Express JS

Express JS is a flexible back-end Node JS framework with a robust web and mobile application feature set. It was released as open source software under the MIT license in 2010. We can design single-page, multi-page, and hybrid web applications and APIs with the Express JS framework. It is built on Node JS, which helps manage routes and servers. And it has been awarded the title of standard server framework for Node JS. It provides several HTTP utility methods for users to easily create APIs and applications.

Check out the following facts about Express JS before continuing:

• Node JS has a 2.4% market share in software frameworks.
• Information technology and services (16%) and computer software (27%) make up the main components of the Node JS industry.
• The top country with Node JS customers is the United States with 48% market share, followed by the United Kingdom and India.
• A Node JS developer can earn an average salary of $61.54 per hour or $120,000 per year in the United States.
• The range varies from 100,000 USD per year for entry level to 150,000 USD per year for experienced professionals.

Why is Express used in web development?

Express JS is becoming a popular choice in web development because it simplifies the process of building web applications using Node JS. Many features are provided by express.js.

Some popular features provided by express.js.

1. Routing:

Express provides routing functionality. Developers can define routes using express with different HTTP verbs (GET, POST, PUT, etc.). It can be associated with different functions. It manages different requests.

Example: The following program demonstrates “How we can define the Routing in Express.js”

2. Middleware:

Middleware functions are a central concept in Node JS. These functions can access ‘req’ and ‘res’ response objects. These objects can be altered before being sent to the route manager. The following program demonstrates Middleware functionality.

3. Error Handling:

Middleware functions are a central concept in Node JS. ‘Req’ and corresponding “res” are the available response objects for these functions. They can modify these objects before reaching the route manager. The following program demonstrates Middleware functionality.

Express JS Interview Questions and Answers for Freshers

1. How do you install an express application generator for scaffolding?

Express application generator is used for rapidly generating an application skeleton. The command given below is used for installing the express application generator.

npm install expr
ess-generator –g
express myApp

It will produce the project “myApp” with some files. Then we install all the dependencies stated in package.json using the command given below:

cd myApp
npm install

2. How do you install a yeoman for scaffolding?

Generators are used by yeoman for scaffolding the applications. And we can use the following command to install yeoman.

npm install –g yeoman

3. Mention the arguments that are available in an Express JS route handler function.

The arguments available in the Express JS route handler function are shown below:

• Res – It is the response object.
• Req – It is the request object
• Next (optional) – This argument is used for passing the management to any of the above-given route handlers.

4. Mention the ways of debugging on Linux as well as Windows.

Debugging on Windows can be done as given below:

set DEBUG = express:*
node app.js

And debugging on Linux can be done as given below:

DEBUG = express:*
node app.js

5. List the built-in middleware functions provided by Express.

Express JS provides the following built-in middleware functions:

1. Static: We use it to serve static assets like images, HTML files, etc.

2. JSON: This is available in Express 4.16.0+. And we use it to pass incoming requests with a JSON payload.

3. Encoded URL: This is also available in Express 4.16.0+. And we use it to forward incoming requests with a URL encoded payload.

6. Mention some third-party middleware provided by Express JS.

Some of the many third-party middlewares that the Express JS provides are given below:

• Cookie-parser
• Body-parser
• Cors
• Mongoose
• Express-validator
• Sequelize.

7. When is application-level Middleware used?

We use the application-level Middleware for binding the app object with the help of the app.use() method. It can be applied on all routes. The syntax is given below:

// This Middleware executes for each route.
App.use(function (req, res, next) {
  console.log(‘Current Time:’, Date.now())
  next()
})

8. Tell us about Router-level Middleware and Built-in Middleware.

Router-level middleware – We use router-level middleware to bind to a specific instance of Express.Router().
Embedded Middleware – Express version 4.x introduces embedded middleware. Connection dependency is eliminated through the use of this middleware.

9. Mention some of the databases with which Express JS is integrated.

Numerous NoSQL and RDBMS databases are supported by Express JS, like:

• MySQL
• MongoDB
• PostgreSQL
• SQLite
• Oracle
• SQLite.

10. What is meant by CORS in Express JS? And what are the ways by which it can be achieved?

CORS stands for Cross-origin Resource Sharing. This allows us to request other servers or domain for certain resources. This task can be accomplished using three methods, one of which is Module Express cors

• Res.header() (or resum.set()) to set multiple header types. The other method is also available for this purpose.
• Res.setHeader(): Only one header can be set this way.

11. What ways are provided by Express JS to configure the properties?

Express JS provides us with two ways for configuring the properties, which are the following:

• With process.ENV
• With require.JS.

12. How are the properties configured with process.ENV?

The properties are configured by the following steps:

• We generate a file within the project folder and name it “.env.”
• All other properties are allowed to be separate within the “.env” file.
• We can engage in any of the properties in server.js.

13. How are the properties configured with require.JS?

The properties are configured by the given steps:

• We generate a file within the config folder of the project folder and name it “config.json.”
• All the config properties are available there within the config.json file.

14. How can the Express JS application be structured?

No specific answer is known for this question. The dimensions of our application and therefore the team involved determine the solution in different situations. The clear logic of Routes and its replacements allows you to embed as many files as you want into any folder structure. The examples given can be read for further inspiration:

• Route map
• Route listings
• MVC vogue controllers.

15. How is the plain HTML rendered?

It’s unnecessary to call the recursion method in order for us to render HTML. Alternatively, we can use the file type that is specified by res.sendFile(‘ And we can use the Express.static() middleware function if we are serving multiple resources from one directory.

Express JS Interview Questions For Experienced

16. Write the code for “Hello world” using Express.

Create a new file by the name- index.js and type the following commands:

var express = require (‘express’);
var app = express ();

app.get (‘/’, function (req, res){
       res.send (“Hello world”);
});

app.listen (3000);

Then go to the terminal after saving it and type:

nodemon index.js

17. What are the most used HTTP methods in Express JS?

The following HTTP methods are most commonly used:

• GET – GET method requests the representation of a specific resource. These queries can only retrieve data.
• POST – Posts the data contained in the request as a new entity made using the POST method. Entity identified by URI.
• PUT – Modification of an existing entity is performed with data contained in the request identified by the URI.
• DELETE – A request to delete the specified source was made using the DELETE method.

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18. How can the cookies be manipulated using ‘Response.cookie()’?

We use the command “res.cookie(‘username’, ‘Flavio’)” for the operation. But it accepts a third parameter containing various options as specified below:

res.cookie (‘username’, ‘Flavio’, { domain: ‘flaviocopes.com’, path: ‘/administrator’, secure: true })
res.cookie (‘username’, ‘Flavio’ , { expires: new Date(Date.now() + 90000), httpOnly: true})

19. When does a Cross-Origin resource get failed in Express JS?

A cross-Origin can fail in the following scenarios-

• If it’s to a different domain
• If it’s to a different port
• If it’s to a different subdomain
• If it’s to a different protocol.

20. How can you use a Pug template engine inside Express?

First we will install it using the given command:

npm install pug

Then we will set it up as follows while initializing the Express application:

const express = require ('express')
const app = express()
app.set ('view engine', 'pug')

21. What do you mean by the sanitizing input process?

People can still type weird stuff through client-side code. They use tools to POST items directly to our endpoint. To do this, Express provides various sanitization methods to avoid these events.

22. Mention some methods for sanitizing.

Consider the following cleanup methods:

• Trim() will remove characters at the beginning as well as the end of the string.
• Escape() will substitute ‘, “, , &, / with the corresponding HTML entities.
• NormalizeEmail() will normalize email addresses.
• Blacklist() will take off characters that appear in the blacklist.

23. Give an example of HTML form code allowing user to upload a file.

Consider the given example of HTML form code that lets a user to upload file.

<form method = “POST” action = “/submit-form”>
  <input type = “file” name = “document” />
  <input type = “submit” />
</form>

24. What are the methods that you can call when the Formidable.File objects arise giving the information about the uploaded file?

We can call the following methods in those scenarios-

• File.name– the name of the file
• File.path– the path to which the file is written
• File.size– the size of the file in bytes
• File. type– the file’s MIME-type.

25. What steps will you follow to set up HTTP for Express with the help of Let’s Encrypt and Certbot?

The given steps are to be followed to set up HTTP:

1. Install certbot
2. Generate SSL certificates with Certbot
3. Authorize Express to serve static files
4. Validate domains
5. Obtain certificates
6. Configure renewals.

26. What options are available in the terminal command for generating a skeleton Express JS app?

The options available are the following:

• –sessions or –s for adding session report
• –hogan or –H for adding Hogan.js engine support
• –ejs or –e for adding EJS engine support
• –css <engine> or –c <engine> for adding style sheet support
• -jshtml or –J for adding JSHTML engine support
• –force or –f for forcing app generation on the directory which is non-empty.

27. What is meant by an event-loop in Node JS?

The event loop processes content asynchronously using listeners and queues. The main thread will dispatch the asynchronous function to another thread every time it needs to be executed. At the same time, v8 is allowed to execute main code. The event loop has different stages including pending callbacks, timers, checks, polling, close callbacks, etc. with different FIFO queues.

28. Why should the Express server and app be separated?

The server initiates middleware, routes, and other application logic. On the other hand, the application contains all the business logic that the server-initiated routes will serve. This allows encapsulating business logic from application logic for smooth operations.

29. What is meant by an Event emitter in Node JS?

It is a Node JS class capable of emitting events. We do this by attaching named events emitted by the object using the eventEmitter.on() function.

30. Differentiate between worker threads and clusters in Node JS.

Cluster:

• Each CPU has a process with IPC to communicate.
• Clusters are useful when multiple servers need to accept HTTP requests through a single port.
• Processes have their own memory because they are distributed across different processors, leading to memory problems.

Worker threads:

• A single process exists with multiple threads.
• Each node contains the one with the most accessible API.
• Memory is shared with other threads.
• We can use it for CPU-intensive tasks.

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Conclusion

Node JS with Express JS, makes it easy to create a complex, high-level web application from just one line of code. This is why it is one of the most sought after skills in the industry today. And here is the complete list of Express JS interview questions that are often asked in interviews. We hope this will be helpful to you in your scheduled interview in the near future.