25+Top Node JS Interview Questions & Answers [UPDATED] | ACTE
Node JS Interview Questions and Answers

25+Top Node JS Interview Questions & Answers [UPDATED]

Last updated on 10th Jun 2020, Blog, Interview Questions

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Node.js is an open-source, cross-platform, JavaScript runtime environment that executes JavaScript code outside a web browser. Node.js lets developers use JavaScript to write command line tools and for server-side scripting—running scripts server-side to produce dynamic web page content before the page is sent to the user’s web browser. Consequently, Node.js represents a “JavaScript everywhere” paradigm, unifying web-application development around a single programming language, rather than different languages for server- and client-side scripts. 

1. What is Node JS?


Node JS is open-source, cross-platform JavaScript runtime environment designed to the execute JavaScript code outside of web browser. Node JS is used for a server-side scripting, unlike traditional web languages that run the client-side. Node JS enables developers to create the scalable and efficient network applications using its unique non-blocking I/O and event-driven architecture. Node JS ensures the speedy execution, making it more suitable for real-time applications that run across the distributed devices as it is built atop Chrome’s V8 JavaScript engine.

2. Why is Node JS single-threaded?


Node JS is a single-threaded because it uses event-driven, non-blocking I/O model, this design choice allows it to the handle multiple concurrent connections efficiently without overhead of managing multiple threads. Developers can leverage the asynchronous nature of a JavaScript to perform tasks without blocking main thread. Callbacks, promises, and async/await are commonly used techniques in the Node JS to manage asynchronous operations.

3. Explain event-driven programming in Node JS?


Event-driven programming in the Node JS refers to design paradigm where the flow of the program is determined by the events, such as user actions, sensor outputs, or messages from the other programs. Events are primarily managed using Event Emitter API in a context of Node JS. When an event occurs, a callback function is executed, making software reactive to the various inputs or occurrences.

4. Explain event loop in Node JS.?


The event loop in the Node JS is a mechanism that allows the Node JS to handle non-blocking operations efficiently. The event loop in the Node JS enables non-blocking I/O operations. The event loop in Node JS allows the Node JS to handle many connections concurrently, making it more scalable for real-time applications. The event loop continuously checks call stack to see if there’s any function to execute at its core. When stack is empty, it looks into event queue to pick up and execute events.

Event loop in Node JS

5. How non-blocking I/O work in Node JS?


Non-blocking I/O in the Node JS allows the system to execute the other tasks while waiting for I/O operations to complete. The system waits for I/O task to finish before moving on to the another task in traditional blocking I/O, but Node JS utilizes event-driven architecture to circumvent this limitation. Node JS continues to handle the other tasks and later retrieves result of that operation using the callback functions when I/O operation is initiated.

6. What are the benefits of using Node JS?


  • Performance and Scalability
  • Rich Ecosystem
  • Versatility
  • Cost-Effective
  • Enhanced Productivity
  • Community Support

7. How do handle errors in Node JS?


Errors in the Node JS are handled using the try-catch blocks, error-first callbacks, event listeners, and promises.

  • For a synchronous operations, try-catch blocks are utilized to the capture exceptions.
  • In asynchronous patterns, error-first callback convention is applied. In error-first callback convention, the first parameter of callback function is dedicated to an error object.

8. How can avoid callback hell?


Use Modularization, Promises, or Async/Await to avoid the callback hell. Callback hell in Node JS is characterized by the series of nested functions or callbacks, which makes code harder to follow and debug.

Modularization : Break down functions into the smaller, reusable modules. This reduces nesting of callbacks and makes code clearer.

Promises : Use Promises, feature in ES6, to represent eventual completion or failure of an asynchronous operation. They simplify chaining of asynchronous functions.

Async/Await : The async/await syntax introduced in the ES8 allows developers to write asynchronous code in the more synchronous fashion, further reducing the need for a nested callbacks.

9. Explain “middleware” in Node JS?


“Middleware” in the Node JS refers to functions executed in middle of a request to a response cycle in a Node JS framework. These functions have access to the request object (req), response object (res), and the next function in application’s request-response cycle. Middleware functions perform a tasks like modifying request objects, handling the authentication, or logging.

10. Differences between process.nextTick() and setImmediate()?


  Criteria process.nextTick() setImmediate()
Execution Timing

Executes before I/O events in the next iteration

Executes after I/O events in the current iteration
Priority in Event Loop Higher priority, runs before other callbacks Lower priority, runs after other I/O event
Use Case Often used for deferring execution to the next tick Useful for executing code after the current event loop.

11. What is npm?


NPM (Node Package Manager) is the tool used for managing and distributing packages (libraries and modules) of reusable code. NPM simplifies a process of incorporating external code into the Node JS projects, streamlining development workflow. NPM packages include the libraries, frameworks, and various dependencies, extending functionality of Node JS applications. NPM allows to manage project dependencies efficiently, facilitating a version control and ensuring application remains consistent and up-to-date.

12. How do create Node module?


  • Create the JavaScript file: Start by creating new JavaScript file (.js) containing a code for module.
  • Define module’s functionality: Write code implementing the functionality want to encapsulate within the module. This includes the functions, variables, or classes.
  • Export module’s functionality: need to export the specific functions, variables, or classes want to expose to make code accessible to the other parts of application. do this using the `module.exports` or `exports` object.
  • Save module: Save JavaScript file containing module code in a directory of choice within the project.
  • Use your module: Require and use module using the `require` function in the other parts of Node JS application.

13. What is package.json?


`package.json` is the important file for Node JS developers as it centralizes the project configuration and dependency management, making it simpler to share, collaborate on, and deploy Node JS applications. package.js file is used to specify the various details including project’s name, version, description, entry point, scripts for the automation, and a list of dependencies required to run project. Developers use package managers like a npm (Node Package Manager) to install the dependencies listed in the `package.json`, making it simpler to manage project dependencies and ensure consistent behavior across the different environments.

14. Explain semantic versioning?


Semantic versioning is the versioning scheme used in Node JS to manage and communicate changes to the software packages and libraries. Semantic versioning consists of the three parts such as major version, minor version, and patch version, written as the “X.Y.Z,” where X, Y, and Z are non-negative integers.

15. How do handle project dependencies?


Node JS developers rely on the package manager like npm (Node Package Manager) or yarn to handle the project dependencies. These tools allow developers to define and document the project’s dependencies in a `package.json` file. Developers can specify the required packages and versions in this file, ensuring consistent and reliable installations across the different environments.

16. Differences between dependencies and devDependencies?


  • Dependencies are the modules or packages essential for a production runtime of the application.
  • DevDependencies are the dependencies required only during development and testing phases of the Node JS project.
  • Dependencies include the libraries and tools that application relies on to function properly.
  • These dependencies are installed using `npm install` command with the `–save` or `-S` flag to ensure they are listed in `package.json` file, making them available for a production use.

17. How do install, uninstall, and update package using npm?


  • Installing a Package: Use command `npm install ` to install package. This command will fetch and install a specified package and its dependencies.
  • Uninstalling a Package: Use `npm uninstall ` to uninstall the package. This command uninstalls specified package from the project.
  • Updating a Package: Use `npm update ` to update the package to its latest version. This command checks for a newer versions of specified package and updates it if available.

18. What is npm shrinkwrap command?


The purpose of the npm shrinkwrap command is to lock down versions of the dependencies used in the Node JS project. The npm shrinkwrap command creates the detailed and unambiguous record of specific package versions that are currently installed in the project, including transitive dependencies.

19. Explain exports and module.exports?


`exports` is the object provided by Node JS to facilitate exporting code from the module. Developers add properties or methods to `exports` object, making them accessible to the other modules requiring this module. It simplifies process of exposing functions, variables, or objects for reuse in the other parts of the application. ‘exports’ is used to the export multiple properties or the functions.

20. How do create a global module?


Develop a Module : Create the module want to make global. This module should contain functionality want to use across the different parts of application.

Export the Module : Use `module.exports` or `exports` object to export functions, objects, or variables want to make available globally.

Make it Global : Use `global` object in Node JS to attach the module to it to make module global.

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    21. What are Callbacks?


    Callbacks are the functions passed as arguments to the other functions and are executed when specific task is completed or an event occurs. Callbacks are used in Node JS for the handling asynchronous operations. Many operations are the non-blocking and rely on callbacks to signal when they are finished in the Node JS, ensuring program to remain responsive. Callbacks are commonly employed with the functions like `fs.readFile`, `http.createServer`, or in an event handling.

    22. What is Callback Hell and how can avoided?


    Callback Hell is the common issue in Node JS development, which occurs when the multiple asynchronous operations are nested within the each other, creating deeply nested and hard-to-read code structure. A Callback Hell situation arises when the callbacks are used extensively, leading code difficult to maintain and understand.

    Avoid Callback Hell :

    Use Promises : Promises provide the more structured way to handle the asynchronous operations. Promises allow to chain asynchronous functions together, making a code more linear and easier to follow. Working with the Promises has become even more straightforward with introduction of the `async/await` syntax.

    Modularize Code : Break down the complex functions into the smaller, reusable modules. This reduces nesting of callbacks and improves a code organization and reusability.

    23. Explain Promises?


    Promises are the objects used for handling asynchronous operations in more structured and manageable way. Promises help avoid the callback hell, making it easier to write and reason about the asynchronous code. Promises allow the developers to chain multiple asynchronous operations together, making a code more readable and maintainable. Promises provide the way to deal with the asynchronous code in a more readable and maintainable fashion.

    24. How are Promises different from Callbacks?


    Promises are objects used for the handling asynchronous operations in more structured and manageable way. Callbacks are functions passed as arguments to the other functions and are commonly used to manage the asynchronous tasks in Node JS. Callbacks rely on the callback pattern, where a function is called once an operation is a complete. While callbacks work well for a simple scenarios, they can lead to callback hell or deeply nested code when dealing with the multiple asynchronous operations.

    25. What are async/await, and how improve code readability?


    Async/await is the feature in Node JS used to simplify the asynchronous code by making it appear more like a synchronous code improving code readability and maintainability. Async/await allows the synchronous style to write asynchronous operations, making code easier to understand. The `async` keyword is used to define a function as a asynchronous, and the `await` keyword is used to pause execution within that function until the promise is resolved. This allows the developers to write code that follows the more natural flow of logic, making it simpler to follow.

    26. How can handle errors with async/await?


    Wrap a Code in a try-catch Block: Wrap the asynchronous code inside the try block. This is the place for asynchronous operation, which leads to error.

    Await Promises : Ensure asynchronous functions that are using a return Promises. This allows to use await keyword to pause execution until Promise is resolved or rejected.

    Handle Errors in Catch Block : Errors will be caught by a catch block if an error occurs during execution of asynchronous code. Handle the error as needed such as logging error, sending the error to an error reporting service, or taking a specific actions based on the error type.

    Rethrow the Error : Choose to the rethrow the error after handling it in catch block depending on application’s requirements. Rethrowing error allows higher-level error handlers to manage error if nece

    27. Explain Event Emitter API?


    The Event Emitter API in the Node JS is a fundamental component that allows the objects to emit named events and register listener functions to respond to those events. The Event Emitter API is the key part of Node JS’s event-driven, non-blocking architecture, used for the handling asynchronous operations.

    28. What are Streams in Node JS?


    Streams in Node JS are abstract interfaces for working with the streaming data in Node JS. Streams in Node JS are used for the handling data in more efficient and memory-conscious manner. Streams in Node JS provide the way to read or write data in chunks, piece by piece. Using the Streams guarantees advantages such as reduced memory usage and faster data processing for a large datasets. Developers are able to handle the data from various sources are files, network requests, or even real-time data sources efficiently by understanding how to work with the streams in Node JS.

    29. Explain different types of Streams?


    Readable Streams : Readable streams are used for a reading data from a source, like a file or HTTP request. Readable streams provide the methods for consuming data chunk by chunk, making them suitable for the handling large datasets without loading everything into the memory.

    Writable Streams : Writable streams are employed for writing a data to a destination, such as file or an HTTP response. Writable streams allow the one to send data in smaller portions, useful for reducing the memory usage.

    Duplex Streams : Duplex streams combine both the readable and writable capabilities. Duplex streams enable the bidirectional communication, making them valuable for tasks like a building network protocols or creating the interactive applications.

    30. How do implement writable stream?


    Require a ‘stream’ module: Require ‘stream’ module in the code using `const stream = require(‘stream’);

    Create a Writable Streamn : Use `Writable` class from the ‘stream’ module to create the writable stream. Create the custom writable stream by extending the `Writable` class and implementing its ‘_write’ method. The ‘_write’ method is responsible for a processing data written to the stream.

    Implement the _write Method : Define how writable stream should handle incoming data inside the ‘_write’ method. Process, store, or transmit a data as needed. Make sure to call `callback` function when have finished processing the data.

    Use the Writable Stream : Use Writable Stream by piping data into it from readable stream once custom writable stream is defined such as using `pipe` method.

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    31. How do create an HTTP server in Node JS?


    Create an HTTP server in the Node JS by using built-in `http` module.

    • Require `http` module in Node JS script.
    • Use the `http.createServer()` method to create HTTP server. This method takes callback function as an argument, executed whenever the request is made to the server.
    • Handle incoming HTTP requests and send responses as needed inside callback function.
    • Specify a port on which the server should listen for the incoming requests using `server.listen()` method.

    32. What are RESTful services?


    RESTful services are the set of architectural principles used for designing a networked applications. RESTful services are the essential for creating scalable, maintainable, and simple to-understand web services.

    33. Explain HTTP verbs.


    HTTP verbs (HTTP methods) are actions performed on resources over the Hypertext Transfer Protocol (HTTP). HTTP verbs define a type of operation requested when making the HTTP requests to a server.

    34. How do handle file uploads?


    The steps involved in the handling file uploads in Node JS applications are :

    • Create the route or endpoint that expects file upload.
    • Set up middleware using the Multer or Formidable to process the incoming files. This middleware parses incoming request, extracts uploaded file, and saves it to a specified location on server.
    • Handle uploaded file as needed, such as storing it in the database, processing it, or serving it to the other users.

    35. How can parse JSON in Node JS?


    Use built-in `JSON.parse()` method to parse JSON in the Node JS. The ‘JSON.parse()’ method plays the pivotal role in converting JSON data into usable JavaScript object. Here are essential steps to deal with the JSON parsing in Node JS.

    Receive JSON Data : Obtain JSON data that is to be parsed. JSON data come from the variety of sources, including the HTTP responses, files, or user input.

    Use JSON.parse() : Pass JSON data as a string to `JSON.parse()` method. The `JSON.parse()` function will then parse string and return a JavaScript object representing the JSON data.

    Working with Object : Access its properties and manipulate a data as needed once have the JavaScript object. This is the point at which extract and practically utilize the information in the Node JS application.

    36. How do handle cookies in Node JS?


    Use `cookie-parser` middleware to handle the cookies in Node JS.

    Installation : Install `cookie-parser` module using npm or yarn by running `npm install cookie-parser` or `yarn add cookie-parser`.

    Require and Initialize : `cookie-parser` module into Node JS application and initialize it.

    Setting Cookies : Set cookies using `res.cookie()` method in a route handlers.

    Reading Cookies : Access them through the `req.cookies` to read cookies.

    37. How enable CORS in Node JS application?


    Use `cors` middleware to enable the Cross-Origin Resource Sharing (CORS) in Node JS application.

    Install CORS middleware : Install the `cors` package using the npm or yarn by using command ’npm install cors’ or ‘yarn add cors’.

    Import and use CORS middleware : Import `cors` middleware into Node JS application and apply it to Express app.

    Configure CORS options : Configure the CORS options based on the specific requirements such as restricting allowed the origins, methods, or headers by passing options object to `cors()`.

    38. How do secure HTTP API in Node JS?


    Authentication : Implement a strong authentication mechanisms like JWT (JSON Web Tokens) or OAuth2 to verify identity of clients trying to access API. Authentication ensures that only the authorized users make requests.

    Authorization : Enforce the proper authorization by defining access control rules. Use the middleware like role-based access control (RBAC) to restrict what each user or client does within HTTP API.

    HTTPS : Always use the HTTPS to encrypt data transmitted between client and server. Node JS provides ‘https’ module for a secure communication.

    Input Validation : Sanitize and validate the user inputs to prevent common security vulnerabilities are SQL injection and cross-site scripting (XSS) attacks. Tools like `the express-validator` help with input validation.

    39. What is Express JS?


    Express JS is the widely used web application framework for a Node JS. Express JS simplifies a process of building robust and scalable web applications and APIs. Express JS provides the set of essential features and tools for a developers such as routing, middleware support, template engines, and range of HTTP utility methods. Express JS follows Model-View-Controller (MVC) architectural pattern, allowing the developers to organize code effectively and separate concerns. Express JS is known for flexibility, performance, and the vast ecosystem of third-party middleware, making it preferred choice for a Node JS developers to create web applications and a RESTful APIs efficiently.

    40. How do create route in Express?


    Import Express : Import Express framework in a Node JS application using `require(‘express’)`.

    Create an Express Application : Initialize an instance of Express application by calling `express()` and storing it in the variable, named `app`.

    Define a Route : Use `app.get()`, `app.post()`, `app.put()`, or `app.delete()` methods, depending on HTTP method want to handle, to define a route.

    41. How do use middleware in Express JS?


    • Import the Express JS framework and create Express application using `const app = express();
    • Middleware functions are the essentially JavaScript functions that take a three arguments: `req` (the request object), `res` (the response object), and `next` (a callback function). Create the custom middleware functions or use an existing ones from Express or a third-party packages.
    • Call `app.use()` to use middleware and pass the middleware function as an argument.

    Middleware in Express JS

    42. How do handle errors in Express JS?


    Here is the list of approaches to effectively handle the errors in Express JS.

    • Middleware for Error Handling: Create the custom error-handling middleware in Express JS using `app.use()` method. Custom error-handling middleware has parameters (err, req, res, and next). Call `next(err)` to pass error to this middleware when an error occurs in any of routes or middleware. Log the error, customize error responses, or perform any necessary cleanup in custom error-handling middleware.
    • Built-in Error Handling: Express provides the default error handler to catch unhandled errors in a web application. Use `app.use()` with a function having four parameters (err, req, res, and next) to create the custom error handler. Ensure this middleware is defined after all other middleware to catch any unhandled errors.

    43. Explain template engines in Express JS?


    Template engines in the Express JS play a pivotal role in simplifying the process of the rendering dynamic web pages. Template engines enable the developers to generate HTML content with the dynamic data seamlessly.

    44. What is Koa JS? How different from Express JS?


    Koa JS is the lightweight and modern web framework for a Node JS that is designed to be more expressive and robust compared to Express JS. Express JS is the widely used web application framework for a Node JS. Express JS simplifies a process of building robust and scalable web applications and APIs.

    45. How do perform input validation in Express JS?


    Use `express-validator` middleware for the input validation in Express JS.

    • Import the `express-validator` library and set it up in the Express application.
    • Create validation rules for a specific routes and endpoints. Specify rules for the parameters, query strings, and request bodies. Example: Check for a required fields, data types, and string lengths.
    • Implement validation middleware in the route handler functions. The validation middleware checks the incoming data against defined rules. Handle errors if validation fails and prevent the further processing.

    46. How do handle file uploads in Express JS?


    Use `multer` middleware to handle file uploads in the Express JS.

    • Install the multer` using npm or yarn. Run ‘npm install multer’ or ‘yarn add multer’ to install ‘multer’.
    • Require `multer` in Express JS application and configure it to specify destination folder where uploaded files will be stored.
    • Set up API endpoint that will receive file upload. Use the `upload.single()` method to handle single file upload or `upload.array()` for the multiple files.
    • Access uploaded file using `req.file` in defined route handler and perform required operations like saving file, validating it, or processing its data.

    47. What are some other Node JS frameworks?


    Express JS : The Express JS is one of the widely adopted Node JS frameworks. Express JS provides the minimal, unopinionated set of features for a building web and mobile applications. Its simplicity and robust middleware support to make it favorite among developers.

    Koa JS : Koa JS is the lightweight and modern web framework for a Node JS, designed by the creators of Express JS. Koa JS uses the ES6 features and async/await, offering a more elegant and efficient way to write a web applications.

    48. How do serve static files in Express?


    Use the `express.static()` middleware, to serve the static files such as HTML, CSS, JavaScript, and images in the Express JS applications. Express checks the specified directory for file when a client makes request for static file and sends it back as response.

    • Import Express framework into Node JS application.
    • Instantiate Express application using `express().
    • Use `app.use()` method with `express.static()` middleware. Specify directory containing a static files inside the `express.static()` function. For example, `express.static(‘public’)` serve files from ‘public’ directory.

    49. How do connect to MySQL database using Node JS?


    • Install the MySQL driver like “mysql” or “mysql2” using npm or yarn. Run command ‘npm install mysql’ or ‘yarn add mysql’ to install MYSQL driver.
    • Require MySQL module in the Node JS application by adding following line at the beginning of script: const mysql = require(‘mysql’);
    • Use `mysql.createConnection()` method to create the connection to the MySQL database by providing database connection details like a host, user, password, and database name.
    • Use `connection.connect()` method to establish connection to MySQL database.
    • Execute the SQL queries and perform database operations using established MYSQL connection.

    50. How do connect to MongoDB database using Node JS?


    • Install MongoDB driver for a Node JS using npm or yarn. Run command ‘npm install mongodb’ or ‘yarn add mongodb’.
    • Import MongoDB module in Node JS application using `require`.
    • Create the connection to the MongoDB server by specifying connection URL and options, if any. Replace `` with the URL of MongoDB server.
    • Perform various operations on MongoDB database within connection callback, like inserting, querying, or updating documents.
    • Close the connection to MongoDB server when done with database operations to ensure that application resources are released properly.

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    51. Explain ORM and ODM.


    ORM(Object-Relational Mapping) is the programming technique that allows the developers to interact with the relational databases using object-oriented programming languages like a JavaScript. ODM is the programming technique that allows the developers to interact with the NoSQL databases like MongoDB. ORM abstracts underlying database structure, enabling the developers to work with the database records as if were regular JavaScript objects. This simplifies database operations, making it simpler to create, read, update, and delete data.

    52. How do execute raw SQL queries in Node JS?


    Use the libraries and modules such as ‘mysql’, ‘pg’ (for PostgreSQL), ‘sqlite3’ (for SQLite), or ‘mssql’ (for a Microsoft SQL Server), depending on database of choice to execute a raw SQL queries in Node JS.

    53. What are Indexes in MongoDB?


    Indexes in the MongoDB are data structures used for an optimizing the querying process in MongoDB database. Indexes act as pointers to actual documents in a collection, allowing for a faster data retrieval. MongoDB indexes improve the query performance by allowing database to quickly locate and access specific documents within collection.

    54. Explain sharding in MongoDB?


    Sharding in the MongoDB is a technique used for distributing the data across multiple machines. Sharding in MongoDB is used to horizontally partition data across the multiple servers or clusters. It is employed to the address scalability challenges and ensure efficient data distribution in the large-scale MongoDB deployments.

    55. How do handle transactions in MongoDB?


    • Create the session using `startSession()` method. This session represents single transaction.
    • Begin transaction within the session using `startTransaction()` method. All the operations within that session will be part of same transaction once the transaction is started.
    • Execute MongoDB operations are inserts, updates, or deletes within transaction using the session.
    • Choose to commit a transaction using `commitTransaction()` to make changes permanent, or abort transaction using ‘abortTransaction()’ to discard any changes made during transaction after performing required operations.
    • End session with `endSession().

    56. How do interact with Redis in Node JS?


    ioredis” library of Node JS to interact with the Redis in Node JS applications.

    • Install ioredis library using npm or yarn. Run command ‘npm install ioredis’ or ‘yarn add ioredis’
    • Require ioredis library inside Node JS application to interact with the Redis.
    • Instantiate the Redis client by providing connection details, such as host and port.
    • Use Redis methods provided by a Redis client to interact with Redis. For example, to set the key-value pair: redis.set(‘myKey’, ‘myValue’);

    57. How do handle database connections in Node JS app?


    Connection Pooling : Employing the connection pooling library, such as `pg-pool` for PostgreSQL or `mysql2` for MySQL, is essential. Connection pooling allows the developers to maintain a pool of database connections that can be reused, reducing overhead of creating and closing connections for the every database operation.

    Environment Variables : Store database connection credentials and configurations as the environment variables. Storing a database connection credentials enhances security and flexibility, as it allows the developers to change database settings without modifying a code.

    58. How do secure database credentials in Node JS applications?


    Environment Variables : Store database credentials like usernames and passwords as environment variables. Storing database credentials prevents them from being hard-coded in a codebase, making it harder for the attackers to access them. Node JS provides `process.env` object to access these variables.

    Use Configuration Files : Separate configuration files which are not committed to the version control. These configuration files are read at a runtime to obtain the necessary credentials.

    Secret Management : Utilize the secret management solution or dedicated service like HashiCorp Vault or AWS Secrets Manager to securely store and retrieve database credentials. These services offer the encryption and access control features to protect the sensitive information.

    59. How do debug a Node JS application?


    Debugging Tools : Node JS offers the built-in debugging tools like the `util.debuglog` module and `–inspect` flag. Use debugging tools to print debug messages or attach the debugger to application.

    Logging : Implement the proper logging using libraries like a Winston or Bunyan. Logging helps track flow of the application and identify issues by inspecting logged data.

    Error Handling : Ensure application code has robust error-handling mechanisms. Catch and log errors appropriately to understand where and why errors occur.

    60. Explain TDD in Node JS?


    Test-Driven Development (TDD) in Node JS is the software development approach where developers write a tests before writing actual code. TDD methodology follows specific cycle: Write minimum code necessary to pass test after writing a failing test case, and finally refactor code if needed. This cycle is repeated iteratively throughout a development process.

    61. What are some testing libraries for Node JS?


    Mocha : Mocha is the widely used test framework, provides the flexible and feature-rich testing environment. Mocha supports the various assertion libraries and runs tests both the synchronously and asynchronously.

    Jest : Jest is the popular testing framework developed by a Facebook. Jest is known for simplicity and speed. It includes the built-in assertion libraries, mocking capabilities, and support for a running tests in parallel.

    Chai : Chai is an assertion library used with the Mocha or other test runners. Chai offers the wide range of assertion styles, making it more customizable and adaptable to the different testing needs.

    62. How do mock dependencies in tests?


    • Select the suitable mocking library like `jest` for an integrated testing framework or `sinon` for standalone mocking.
    • Import module or function want to test and its dependencies.
    • Use mocking library to replace the actual dependencies with mock objects or the functions. For example, with `jest`, use `jest.mock()` to replace dependencies.
    • Define behavior of the mock objects or functions. Specify a return values, simulate errors, or track function calls and arguments.
    • Execute tests. The mocked dependencies behave as a specified, allowing to focus solely on unit under test.

    63. How do test asynchronous code in Node JS?


    • Select the testing framework compatible with the Node JS, such as Mocha. Install it using the npm or yarn.
    • Create test cases describing expected behavior of asynchronous code.
    • Employ assertion libraries are Chai or Jest’s built-in assertions to verify expected outcomes of asynchronous functions.
    • Use callbacks or Promises in a test cases to handle asynchronous operations depending on a code’s style.
    • Make sure to handle code properly in test cases using `async` and `await` if code uses async/await.
    • Consider using the libraries like `sinon` to create mocks, stubs, and spies for a testing scenarios involving timers or external services.
    • Execute tests using testing framework’s command line or configuration.

    64. What is spy in testing?


    A spy in testing is the specialized testing utility or function that allows the developers to observe and track behavior of functions or methods during the execution of a code. Spy is used to monitor whether certain functions have been called, how many times called, with what arguments, and sometimes even what they returned.

    65. What is stub in testing?


    A stub is the testing technique or a component used to simulate behavior of certain parts of the code during testing. Stubs are used to isolate and control the testing environment, ensuring focus remains on a specific piece of code being tested without being influenced by the external dependencies. Stubs are the fundamental part of unit testing and are invaluable in the ensuring the reliability and robustness of Node JS applications. Stubs are used to replace the external functions, methods, or modules with the simplified versions returning predetermined responses. These simplified versions, or stubs, allow developers to test code in isolation and verify a code to function correctly even when interacting with the external components.

    66. How do performance testing for Node JS application?


    Define Test Objectives : Define performance testing objectives clearly. Determine what aspects of application’s performance are to be evaluated, such as response time, throughput, resource utilization, or scalability.

    Select Testing Tools : Choose appropriate performance testing tools are Apache JMeter, Artillery, or LoadRunner. Testing tools help to simulate real-world user traffic and measure application’s response under the different loads.

    Create Test Scenarios : Develop the realistic test scenarios resembling actual user behavior and traffic patterns. Consider the scenarios for a various use cases, such as login, data retrieval, and transaction processing.

    67. Explain hot-reloading?


    Hot-reloading is the fundamental concept in Node JS development, which involves the dynamically updating an application while the application is running, without the need to restart entire server or application. This capability significantly enhances development process by allowing an immediate changes in their code without any interruptions.

    68. How do handle logging in Node JS?


    The Console Module : Node JS has built-in ‘console’ module offering the various methods like `console.log()`, `console.error()`, `console.warn()`, and more. Console module functions are used to print the messages to console for debugging and logging purposes.

    Choose Log Levels : Use log levels to categorize a log messages. Common log levels are include ‘info,’ ‘warn,’ ‘error,’ and ‘debug.’ Every level serves a specific purpose, and help in prioritizing and filtering log messages.

    Implement Logging Libraries : Use the logging libraries like Winston, Bunyan, or Morgan for more advanced logging capabilities. Logging libraries are provide features log rotation, file logging, and log formatting.

    69. How is Socket.IO different from WebSocket API?


    Socket.IO is designed to simplify a real-time communication and offers features like the automatic reconnection and fallback mechanisms. Socket.IO is used both on server side (Node JS) and client side (JavaScript). WebSocket API is the low-level protocol that operates directly over TCP, providing simple and efficient way to exchange data in a real time. The WebSocket API is native to modern the web browsers and does not require any additional libraries, it involves the more manual handling of the connection management and error handling compared to Socket.IO.

    70. How do handle real-time data in Node JS?


    • Use technologies like a WebSockets or the `Socket.IO` library to handle real-time data in Node JS. These options enable the bi-directional communication between the server and clients in a real time, ensuring an instant data updates and interactions.
    • Socket.IO simplifies a real-time data handling by providing a robust framework for managing the WebSocket connections
    • Leverage Node JS for building the RESTful APIs or GraphQL endpoints to facilitate a real-time data retrieval and updates, to ensure application remains responsive and data stays synchronized across the connected clients.

    71. How do scale a real-time application?


    • Use load balancer to distribute incoming traffic across the multiple server instances. Use load balancers to ensure no single server becomes the bottleneck, enabling horizontal scalability.
    • Leverage microservices architecture to break down application into smaller, manageable components. Breaking down application into the manageable components promotes easier scaling of a specific functionalities and allows for a better resource allocation.
    • Utilize the caching layer like Redis to enhance a real-time application scalability by reducing the database load and improving response times.

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    72. How do ensure message delivery in real-time apps?


    WebSocket Protocol : Use a WebSocket protocol to establish persistent, two-way communication channel between server and clients. WebSocket enables real-time data transfer and immediate message delivery without overhead of HTTP requests.

    Message Queues : Implement message queuing systems are RabbitMQ or Redis Pub/Sub. Message Queuing systems allow the developers to store and manage messages, ensuring messages are delivered to intended recipients even if they are temporarily offline.

    Acknowledgments : Implement acknowledgment mechanisms where sender receives confirmation when message is successfully delivered to recipient. The message is resent until delivery is confirmed if no acknowledgment is more received.

    73. How do handle real-time session management?


    Use ‘express-session’ middleware along with the WebSocket or Socket.IO to handle the real-time session management. The approach involves below-mentioned steps.

    Set up express-session : Configure and install `express-session` in Express.js application. ‘express-session’ middleware creates and manages user sessions. Specify options such as session storage, cookie settings, and more as needed to manage sessions.

    Integrate WebSocket : Integrate the WebSocket library like `ws` or Socket.io into Node JS application to achieve real-time communication. These libraries provide the WebSocket server functionality that allows the bidirectional communication between server and connected clients.

    74. How deal with latency issues in real-time apps?


    Codebase Optimization : Optimize Node JS codebase. Ensure the application is efficiently written, with the well-structured code and minimal resource consumption. This includes the optimizing database queries, reducing the unnecessary computations, and managing memory efficiently.

    Caching : Implement the caching mechanisms to store frequently accessed data in a memory. This reduces need to repeatedly fetch data from a database, improving response times.

    Load Balancing : Employ the load balancing techniques to distribute incoming traffic across the multiple server instances. This helps prevent overloading a single server and ensures requests are evenly distributed, reducing latency.

    75. How do secure WebSocket connections?


    Use ‘wss’ (WebSocket Secure) for encrypted connections instead of ‘ws’, which is standard WebSocket protocol. ‘Wss’ protocol ensures the data transmitted between client and server is encrypted using the SSL/TLS, preventing eavesdropping.

    76. How do handle disconnected users in real-time app?


    Implement reconnection strategies : Use libraries or built-in mechanisms to enable the automatic reconnection for disconnected clients. This ensures the users to quickly resume activities upon reconnection.

    Use heartbeat checks : Regularly check connection status of clients through the heartbeat messages. Mark the time frame as disconnected if client doesn’t respond within the specified time frame and clean up its resources on a server.

    Implement session management : Store user session data in the reliable and persistent database to maintain the user state even after disconnection. Make sure the user’s session data is restored, providing seamless experience when user reconnects.

    77. How do deploy a Node JS application?


    • Ensure application code is well-structured and free from the any errors or bugs. Perform thorough testing to the validate its functionality.
    • Choose hosting provider or platform that suits project’s requirements. Popular options include the AWS, Heroku, or VPS providers like DigitalOcean.
    • Prepare deployment environment by installing Node JS and npm on hosting server, if not already available.
    • Use the version control system like Git to manage code and deploy it to server. This helps with simple updates and rollbacks.
    • Set up any necessary configurations, environment variables, and security measures for application on hosting server.

    78. How do manage environment variables in Node JS?


    Managing environment variables in the Node.js application is crucial for maintaining security and configurability of apps. Manage Environment Variables in Node JS applications using ‘process.env’ object.

    79. How do monitor Node JS application?


    Implement Logging : Implement comprehensive logging within Node JS application using libraries areWinston or Bunyan. Log relevant events, errors, and performance data to gain insights into behavior.

    Use Application Performance Monitoring (APM) Tools : Employ APM tools are New Relic, AppDynamics, or Datadog to gain a real-time visibility into the application’s performance.These tools help to track the response times, errors, and resource consumption.

    Set Up Error Handling : Implement the robust error handling mechanisms within code to catch and log exceptions. This ensures developers are alerted to any unexpected issues.

    80. How do handle uncaught exceptions in Node JS?


    Use ‘process.on(‘uncaughtException’)’ event listener to handle uncaught exceptions in the Node JS. ‘process.on(‘uncaughtException’)’ event is triggered when an exception occurs in application that is not caught by `try-catch` block. Use this event to log error and gracefully exit the application.

    81. How do achieve scaling in Node JS?


    Load Balancing : Implement a load balancing techniques using tools like the NGINX or built-in clustering modules in the Node JS.

    Horizontal Scaling : Scale horizontally by adding the more servers or instances to the application.

    Microservices Architecture : Decompose application into smaller, independent microservices to be individually scaled.

    Caching : Implement the caching mechanisms, such as Redis or Memcached, to store frequently accessed data in a memory.

    82. How do perform load balancing in Node JS?


    Reverse Proxy Servers : Use the reverse proxy servers like Nginx or HAProxy. Reverse Proxy Servers sit in front of Node JS application servers and distribute incoming requests based on the predefined rules. They balance load by evenly distributing requests among the multiple Node JS instances, improving the reliability and performance.

    Node JS Clustering : Node JS provides the built-in `cluster` module to create the multiple worker processes, each running Node JS application. These worker processes share a same port, effectively spreading load. This approach utilizes all available CPU cores and is particularly useful for the CPU-bound tasks.

    83. How do use PM2 in Node JS?


    PM2 is the process manager for Node JS applications that helps in managing, monitoring, and keeping Node JS applications running efficiently.

    Installing PM2 : Make sure have PM2 installed globally on system. Install the PM2 process manager using the npm by running following command: npm install pm2 -g

    Monitoring and Managing Applications : PM2 provides the various commands to monitor and manage applications.

    Managing Start-up Scripts : Make sure Node JS application runs automatically on a system startup.

    Logging and Monitoring : PM2 offers the features for logging and monitoring Node JS applications. Check logs using a below command.

    84. How do perform clustering in Node JS?


    • Require `cluster` module in a Node JS application.
    • Use `cluster.isMaster` property to determine if current process is the master process. The master process is responsible for the managing worker processes.
    • Fork multiple worker processes using `cluster.fork()` if it’s master process. Each worker process runs application code. The Node JS cluster module automatically handles the load balancing by distributing incoming requests across worker processes. It is not needed to implement the custom load-balancing logic.

    85. How do handle process crash in Node JS?


    Use Cluster Module : Node JS provides Cluster module to create the multiple child processes (workers) to share a same server port. Other processes continue to handle incoming requests if one of processes crashes. Using Cluster Module ensures the high availability and fault tolerance.

    Implement process Monitoring : Implement the process monitoring tools like PM2, Forever, or systemd to automatically restart Node JS application if it crashes unexpectedly. Process Monitoring tools provide the logging and resource management features.

    86. How do you prevent NoSQL injection attacks?


    Utilize parameterized queries when interacting with NoSQL database, such as MongoDB to prevent NoSQL injection attacks in the Node JS application. Parameterized queries ensure user inputs are treated as data and not executable code. Parameterized queries replace any input values with the placeholders and then bind actual values to those placeholders, preventing the malicious input from affecting the query’s structure. This practice ensures even if an attacker tries to inject malicious code, it will not be executed by database.

    87. Explain the role of the Express.js framework in Node.js development.


    Express.js is a web application framework for Node.js. It simplifies the process of building robust web applications by providing a set of features for routing, middleware, and template engines.

    87. Explain the role of the Express.js framework in Node.js development.


    Express.js is a web application framework for Node.js. It simplifies the process of building robust web applications by providing a set of features for routing, middleware, and template engines.

    88. What is the significance of the EventEmitter class in Node.js?


    EventEmitter is a core module in Node.js that facilitates the implementation of the observer pattern. It allows objects to emit custom events and enables other objects to listen and respond to those events.

    89. What is npm, and how does it contribute to Node.js development?


    npm in Node.js is a package manager that facilitates dependency management by allowing developers to define and track project dependencies in the package.json file. It streamlines the installation of external packages, making it easy to integrate libraries and modules. npm also provides a platform for discovering, sharing, and distributing Node.js packages globally. Developers can use npm commands to install, update, and remove packages, enhancing the modularity and maintainability of Node.js projects. The package.json file, generated and managed by npm, serves as a manifest for project configuration and dependencies.

    89. What is npm, and how does it contribute to Node.js development?


    npm in Node.js is a package manager that facilitates dependency management by allowing developers to define and track project dependencies in the package.json file. It streamlines the installation of external packages, making it easy to integrate libraries and modules. npm also provides a platform for discovering, sharing, and distributing Node.js packages globally. Developers can use npm commands to install, update, and remove packages, enhancing the modularity and maintainability of Node.js projects. The package.json file, generated and managed by npm, serves as a manifest for project configuration and dependencies.

    90. How does npm in Node.js simplify dependency management and package installation?


    npm in Node.js functions as a package manager, enabling streamlined dependency management. Developers define and track dependencies in the package.json file, and npm simplifies the installation of external packages. It supports global sharing and distribution of Node.js packages. Additionally, npm commands facilitate package installation, updates, and removal, enhancing project modularity. The package.json file, managed by npm, serves as a configuration manifest for project dependencies and settings.

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