12 Best Node.js Interview Questions & Answers

Looking for the best node.js interview questions and answers to help you get prepared for your upcoming technical interview? Look no further. Node.js has become one of the most popular programming languages to use in production applications over the past five years. Because of its reliable and scalability, it has quickly eclipsed options like Ruby on Rails or even PHP. Though those other frameworks are very dependable as well, using Node.js allows engineers to build lightweight applications that can have considerable logic. This means, more functionality, more customization at a cheaper price for the user and their bandwidth.

Why should you study Node.js interview questions?

Node.js is javascript written for server-side applications. Because of its fast learning curve, dynamic-typing, and features focused on rapid development, Node.js is an extremely popular tool in the tech community. Node.js software has been written at tech giants like Facebook, Google, Microsoft, and Uber to power their products and delivers features to users. As a backend engineer, a full-stack engineer, or even as a front-end engineer applying for positions within technology companies, working knowledge of Node.js and what it is used for is an enormous benefit. These Node.js questions are written to give you a practical understanding of Node.js and its many use cases.

Node.js interview questions & answers

1. Describe what Node.js is used for in a software application.

Node.js is javascript written on the server of an application, which powers the same types of logic and behaviors you might expect from a language like PHP, C++, or Java. Node.js can be used to run any sort of server-side application logic. For example, reading and writing from databases, connecting to external APIs or microservices to consume or submit data, calculate algorithmic logic or render HTML pages to deliver to a users’ browser. Node.js is dynamically typed, just like javascript, so it often has an easier learning curve for developers who are used to writing code in a front end development environment.

2. Can Node.JS be used with a RDBMS system (Relational Database System)?

Yes. Just like any other server-side language, Node.js can be used to perform reads and writes to an RDBS system like MySQL, PostgreSQL, etc. There is a large ecosystem of libraries and plugins which make this behavior simpler for the developer.

3. Explain a tool like express.js, and how it relates to Node.js.

Express.js (as well as its alternatives, like Koa and Koa2.js for example), provide easy abstractions for developers who want to write applications with an MVC (Model View Controller) architecture.

In the most basic sense, these tools map HTTP traffic to your server to function calls that you have written in javascript. For instance, express.js will allow you to route a URL like http://my.api/users to a function called getUsers() which would retrieve a list of all the users stored in your application data-storage.

4. Is Node.js good for writing computationally expensive code (code that takes a long time to run)?

Node.js can be used to write long-running code; however, an engineer must be careful in doing so. Using a basic architecture, Node.js is not an ideal tool to run CPU intensive code because of its single threaded design.

For example, imagine as a Node.js developer you create a method that takes 15 seconds to execute. If two users invoke that method simultaneously, the logic will not run concurrently. Instead, the second invocation will begin after the first 15-second invocation has completed. 10 such requests result in 150 seconds of computation for your Node.js application. Yikes!

This can have even more unintended consequences, and a method with 15 second invocation time will also block all other operations that must be executed to your server. If 100 users execute a .01s register() function, those functions too will be executed at the conclusion of the initial 15-second invocation.

This problem should be avoided by keeping the logic that is executed by Node.js functions simple and computationally inexpensive.

5. How can the blocking behavior caused by Node.js’s single-threaded architecture be overcome?

Fortunately, Node.js offers a concept of “child_process”. By spawning a child process, computationally expensive code can be run outside of the primary thread. In this way, we avoid a scenario where our application is stuck behind a blocking operation on the main thread.

However, even this approach is limited. Each child_process occupies a significant amount of memory and resources. For example, in one study while the primary Node.process occupied 14mb of memory, each child process occupied 7mb of memory. In the case that hundreds of these child-processes would need to be handled at once our Node server could easily crash.

One alternative is to run these computationally expensive processes in outside microservices which run with multi-threaded languages and then return their results to the Node.js application.

6. What is a good organizational reason to use Node.js

There are a few potential reasons to use Node.js in your software architecture.

First, Node.js is widely known to be a great tool for building features fast and delivering them to customers. Due to its dynamic typing, easy environment setup, and a wide ecosystem of convenience tools it is often possible to launch working prototypes to users within very short time-scales.

Node.js also lends itself to cross-team development. If you have a group of front-end engineers in your organization (as is likely for most software organizations), these people are useful resources for backend engineering as well -- if you are using Node.js. Because of the identical syntax, front end javascript developers who are used to writing code in React.js, Angular, or simply Vanilla JS will have a mild learning curve to pick up and understand Node.js development.

7. What are some options for introducing type-safety into your Node.js applications.

Introducing type-safety to a Node.js application can have massive benefits in terms of code-quality, maintenance, reliability, modularity, and more. In general, type systems are often argued to improve codebases and developer abilities.

Tools like Flow.js and Typescript are tools that can be used for defining the data-types inside your Node application. This means that during the build process, any type-inconsistencies will alert the developer that the build has failed. Although this may initially sound like an inconvenience for developers, it offers an awesome benefit to software reliability because all of these errors are caught at build time before users have the chance to see errors in the software at run time while they use it.

8. When writing a function in a typically Node.js callback handler, what is the first argument passed to the function?

A callback function in a Node.js application is a function which is called usually after some asynchronous operation has completed. For example, perhaps an HTTP request is made to an external service via the `request` method.

In this case, the first argument supplied to the callback function is typically an `error` object. This error object can be caught conditionally, in order to throw an exception or to log useful information to the user.

9. What is an integration test; how would integration tests be used in the context of a Node.js application?

Integration tests are used to test the interactions between models in your application. Integration tests differ from unit tests, which are aimed at testing a single module in isolation, meaning, in a unit test all external dependencies are “stubbed”. (Faked, in similar terms.) However, in an integration test, each module depends on its actual dependencies returning the values which are expected, thereby both modules are guaranteed to communicate successfully and as expected while users use the application.

10. What is async/await? How can using async/await improve readability of Node.js applications?

Async/await is a feature available in Node.js versions >7.6, which allows developers to write logic for asynchronous behavior in a more legible way than using callback functions or promises. These code snippets show how asynchronous behavior might be handled in a node application with callbacks and async/await, respectively.



11. Describe the package.json, why is it important in a Node.js project?

package.json is a file common to both Node.js and front-end javascript applications like those written in React.js or Angular.

This file is used to define the various properties of your project. Some of these properties include the authors’ names or organization name, the version of the currently released project, a reference to where the source code can be found in a version control system, as well as licensing information for the software. (For example, your software might be open-source or carry a license like MIT which would be listed in the package.json file).

Perhaps most importantly; however, is that package.json lists the dependencies for your project and their versions. By listing your project dependencies in package.json, future developers can download your package source code from version control, and then write the command npm install into their terminal in order to quickly download all the dependencies to their local filesystem. This is vastly superior to installing dependencies and then checking the dependencies into your version control system.

12. Can you write a RESTful API using Node.js? What would be some of the general steps taken towards doing so?

Yes. Because Node.js is the javascript programming language for server-side applications, and a RESTful API is a server-side application; this is a perfect application.

Generally, a RESTful API is a set of mappings between HTTP endpoints (e.g. https://www.my-api.com/api/{resouce} and some business logic which will ultimately respond with a payload of data.

With all that in mind, there are a few essential steps most developers would consider when building a RESTful API with Node.js.

In terms of tools, the engineer would want to choose a framework for routing HTTP requests like express.js or koa.js. The engineer would also want to investigate the types of data storage solutions that would best support his API. If the structure of data is uncertain, perhaps a NoSQL database like MongoDB would be better than an RDBMS like MySQL.

Another important step in designing a RESTful API with Node.js, as well as with any other programming language, is to decide upon the set of HTTP endpoints which will support retrieving and submitting data to the API.


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