Node.js Interview Questions

What is Callback?

A callback is a function called at the completion of a given task; this prevents any blocking, and allows other code to be run in the meantime. Callbacks are the foundation of Node.js. Callbacks give you an interface with which to say, "and when you're done doing that, do all this."

Are there any disadvantages to using Node.js?

A multi-threaded platform can run more effectively and provide better responsiveness when it comes to the execution of intensive CPU computation, and the usage of relational databases with Node.js is becoming obsolete already.

Differentiate between Angular and Node.js

Angular is used by JavaScript developers to develop client-side interactive web applications, whereas Node.js is used to develop fast and scalable network and server-side applications.

Explain Error first callback in Node.js.

Answer: Error-first Callback is a function in Node.js used to pass errors and data. The callback is function asynchronous in nature, called after the Ajax request is completed. The error-first Callback function has the first argument as an error object and the second argument is response data that returns on successful response without any error.

What is Callback Hell and what is the main cause of it?

Asynchronous JavaScript, or JavaScript that uses callbacks, is hard to get right intuitively. A lot of code ends up looking like this: See the pyramid shape and all the }) at the end? This is affectionately known as callback hell. The cause of callback hell is when people try to write JavaScript in a way where execution happens visually from top to bottom. Lots of people make this mistake! In other languages like C, Ruby or Python there is the expectation that whatever happens on line 1 will finish before the code on line 2 starts running and so on down the file.

What are the benefits of using Node.js

Aynchronous and Event Driven - All APIs of Node.js library are aynchronous that is non-blocking. It essentially means a Node.js based server never waits for a API to return data. Server moves to next API after calling it and a notification mechanism of Events of Node.js helps server to get response from the previous API call. Very Fast - Being built on Google Chrome's V8 JavaScript Engine, Node.js library is very fast in code execution. Single Threaded but highly Scalable - Node.js uses a single threaded model with event looping. Event mechanism helps server to respond in a non-bloking ways and makes server highly scalable as opposed to traditional servers which create limited threads to handle requests. Node.js uses a single threaded program and same program can services much larger number of requests than traditional server like Apache HTTP Server. No Buffering - Node.js applications never buffer any data. These applications simply output the data in chunks.

What are differences between process.nextTick() and setImmediate() function?

Both functions control the order of code execution in the event loop. Callback handlers are scheduled in the event queue in both of these functions. setImmediate(): Scheduled callbacks are executed in Check handlers phase of event loop when setImmediate() function is applied. Recursive calls to setImmediate() will not block the event loop, the call is executed on the next event loop iteration. process.nextTick(): Scheduled callbacks are processed at starting of the event loop and between each phase of the event loop when process.nextTick() function is applied. Callbacks to process.nextTick() are resolved before continuation of the event loop, blocking event loop if this function is called

Explain Buffer class in Node.js

Buffer class in Node.js provides a way to manage binary data streams. Unlike Unicode, JavaScript does not support binary data. To process TCP streams or file systems, it is important to handle octet streams. Buffer class in Node.js offers instances that store raw data and allocate raw memory outside V8 heap.

What is the most suitable database used along with Node.js?

Cassandra, CouchDB, MySQL, MongoDB, Neo4j, Oracle, PostgreSQL, and ElasticSearch are some of the databases used along with Node.js. We can install the connection drivers for these Databases with the command npm install driver_name. However, MongoDB is more suitable for back-end management with Node.js.

How Crypto is used in Node.js?

Crypto module of Node.js contains OpenSSL's hash, HMAC, cipher, decipher, sign and verify functions. An algorithm created by the crypto module is used for data encryption and decryption, for storing passwords in the database in encrypted form.

Explain EventEmitter in Node.js

Custom events can be handled using EventEmitter class from the Events module. In the case of an e-commerce application, when your payment is successful or declined, the user should receive an email informing the status, so interaction with payment gateway i.e. the result should be sent as an email so callback is added to the email event. There are many more operations that can interact with each other when the payment request is carried out. With event emitter, these events are handled using the following code: eventEmitter.on('SendMail', listener);

How DNS module is used in Node.js?

DNS module is used for functionalities, such as Domain Name System (DNS) lookup and operating system name resolution. DNS is like a website address. For example, www.yahoo.com is converted into IP Address (202.165.107.50).

Explain Event Loop in Node.js.

Event Loop is a mechanism that lets Node.js continue performing I/O operations to the system kernel without any interruption. Event loop is initialized by Node.js that processes input script making async API calls, call process.nextTick() or schedule timers followed by event loop processing. Event loop process timers, pending callbacks, remain idle, prepare for poll for incoming data and connections, checks for next request, and close callback till next request.

List some of the Node.js libraries often used?

Express: It is a web framework for node.js Socket.io: It is for event-based real-time communication Cors: Node.js pack to provide connect/express middleware Passport: It is used to authenticate requests with strategies extensible plugins Axios: HTTP client who is Promise based for node.js and browser Multer: Handles multipart/form-data and file uploads Morgan: A Node.js middleware that is an HTTP request logger HTTP: errors - It helps to generate HTTP errors for Connect, Koa, and Express

What are the asynchronous tasks that should occur in an event loop?

Following are some of the tasks that can be done using an event loop asynchronously: Blocking send requests High computational requirement Real-time I/O operations

List functionalities of some of the Node.js core modules.

HTTP module contains classes, events, and methods to create Node.js HTTP server. URL consists of methods for URL parsing and resolution. fs module has classes, events, and methods that work with file I/O. Path contains methods to deal with file paths. util is the utility functions useful for programmers that are included in util module. querystring module consists of methods to deal with the query string.

What is "callback hell" and how can it be avoided?

However, these approaches are pretty dated at this point. The current solution is to use async/await—an approach that leverages Promises and finally makes it easy to flatten the so-called "pyramid of doom" shown above.

What do you mean by the term I/O?

I/O is the shorthand for input and output, and it will access anything outside of your application. It will be loaded into the machine memory to run the program, once the application is started.

What does event-driven programming mean?

In computer programming, event driven programming is a programming paradigm in which the flow of the program is determined by events like messages from other programs or threads. It is an application architecture technique divided into two sections 1) Event Selection 2) Event Handling.

What is Node.js Libuv library and its uses?

Libuv is asynchronous input/output that comes as libraries with Node.js installation. Various features Libuv has are listed below: Asynchronous - TCP & UDP sockets - DNS resolution - File and file system - Thread poo - lSignal handling, Backed with the full-featured event loop Child processes File System Events

Describe NPM and its functionality in Node.js

Node Package Manager (NPM) is an online repository of JavaScript libraries that has more than 350,000 packages that can be utilized to build efficient applications and Node.js projects with ease. It is a command-line utility for accessing an online repository that facilitates inversion and dependency management and package installation. Various features can be searched from this repository here. Once found you can install it on your client machine by running the command - npm install feature_file. You can use the feature by using require('./feature_file') in JavaScript code.

How assert is used in Node.js?

Node.js Assert modules are used for function assertion. Assert function works for verifying invariants, returns nothing in output if the condition is true else assertion error is displayed by console.

What is typically the first argument passed to a Node.js callback handler?

Node.js core modules, as well as most of the community-published ones, follow a pattern whereby the first argument to any callback handler is an optional error object. If there is no error, the argument will be null or undefined. A typical callback handler could therefore perform error handling as follows:

How to avoid Callback Hell in Node.js?

Node.js internally uses a single-threaded event loop to process queued events. But this approach may lead to blocking the entire process if there is a task running longer than expected. Node.js addresses this problem by incorporating callbacks also known as higher-order functions. So whenever a long-running process finishes its execution, it triggers the callback associated. Sometimes, it could lead to complex and unreadable code. More the no. of callbacks, longer the chain of returning callbacks would be. There are four solutions which can address the callback hell problem: Make your program modular - It proposes to split the logic into smaller modules. And then join them together from the main module to achieve the desired result. Use async/await mechanism - Async /await is another alternative for consuming promises, and it was implemented in ES8, or ES2017. Async/await is a new way of writing promises that are based on asynchronous code but make asynchronous code look and behave more like synchronous code. Use promises mechanism - Promises give an alternate way to write async code. They either return the result of execution or the error/exception. Implementing promises requires the use of .then() function which waits for the promise object to return. It takes two optional arguments, both functions. Depending on the state of the promise only one of them will get called. The first function call proceeds if the promise gets fulfilled. However, if the promise gets rejected, then the second function will get called. Use generators - Generators are lightweight routines, they make a function wait and resume via the yield keyword. Generator functions uses a special syntax function* (). They can also suspend and resume asynchronous operations using constructs such as promises or thunks and turn a synchronous code into asynchronous.

What is node.js?

Node.js is a Server side scripting which is used to build scalable programs. Its multiple advantages over other server side languages, the prominent being non-blocking I/O.

Explain the working of Node.js.

Node.js is a runtime environment that has V8 - JavaScript engine, NPM package manager for Node.js, and Libuv libraries. V8 is a JavaScript engine that converts browser JavaScript and Node.js code into machine code, making JavaScript run everywhere. NPM is a package manager- a package repository that contains library files containing features that can be included in Node.js. Real-time applications installed on mobile that displays real-time information on request such as Market share prices, or availability of flights, movie tickets, etc. need data (current status). This data can be retrieved from any server using a single thread of Node.js using non-blocking I/O, wherein waiting for information is avoided, can attend multiple requests simultaneously. Node.js asynchronous feature is utilized when the response from another server returns with the required information, wherein callback function is executed. Hence Node.js is applicable for I/O intensive applications. Further, to deal with multiple requests, Node.js uses a concept called Libuv - libraries built in C language. These libraries use a system kernel that uses multiple threads for these requests making Node.js run fast.

What is Node.js?

Node.js is a very popular scripting language that is primarily used for server-side scripting requirements. It has numerous benefits compared to other server-side programming languages out there, the most noteworthy one being the non-blocking I/O.

Briefly explain the working of Node.js.

Node.js is an entity that runs in a virtual environment, using JavaScript as the primary scripting language. It uses a simple V8 environment to run on, which helps in the provision of features like the non-blocking I/O and a single-threaded event loop.

where is Node.js used?

Node.js is used in a variety of domains. But, it is very well regarded in the design of the following concepts: Network application Distributed computing Responsive web apps Server-Client applications

How node.js works?

Node.js works on a v8 environment, it is a virtual machine that utilizes JavaScript as its scripting language and achieves high output via non-blocking I/O and single threaded event loop.

Why is Node.js single-threaded?

Node.js works on the single-threaded model to ensure that there is support for asynchronous processing. With this, it makes it scalable and efficient for applications to provide high performance and efficiency under high amounts of load.

How does Node.js handle child threads?

Node.js, in its essence, is a single thread process. It does not expose child threads and thread management methods to the developer. Technically, Node.js does spawn child threads for certain tasks such as asynchronous I/O, but these run behind the scenes and do not execute any application JavaScript code, nor block the main event loop. If threading support is desired in a Node.js application, there are tools available to enable it, such as the ChildProcess module.

How does Node.js handle Child Threads?

Node.js, in its essence, is a single thread process. It does not expose child threads and thread management methods to the developer. js does spawn child threads for certain tasks such as asynchronous I/O, but these run behind the scenes and do not execute any application JavaScript code, nor block the main event loop. If threading support is desired in a Node.js application, there are tools available to enable it, such as the ChildProcess module.

Explain reactor design pattern of Node.js

Nonblocking I/O feature is due to reactor pattern, before I/O request is generated, the handler is submitted to a demultiplexer that handles concurrency, collecting requests as events, and executes these events in queues, thereby avoids I/O blocking. Reactor pattern consists of following: Resources - Multiple applications with I/O operations shares this resource. Event Notifier (Synchronous) - It pushes new events in the event loop. Event Loop - New events occur with the event handler with the event loop. Request Handler - Handlers are used for events registered on resources provided by the application.

Explain Event-driven programming.

Programming based on user interactions on the graphical user interface is called Events; like clicking the submit button, selecting an option from the radio button, typing a text into the text field, uploading the image file. Event handler or listener that can also be a callback is a method that is called. It accepts user input as parameters and performs some tasks on user and browser actions like page load, HTML page popup, etc.

Explain REPL with context to Node.js

REPL is an environment of a computer wherein the system responds with an output to the entered command. Some of the tasks are: Read: As the name indicates, user input is read and converted into JavaScript data structure and then stored in memory. Eval: Receives and evaluates data structure. Print: Final output is printed Loop: Command is looped until CTRL + C is pressed twice.

What is REPL? What purpose it is used for?

REPL stands for (READ, EVAL, PRINT, LOOP). Node js comes with bundled REPL environment. This allows for the easy creation of CLI (Command Line Interface) applications.

What are modules in Node.js? Explain with few examples.

Simple or complex functionalities are grouped as Modules in Node.js Core Modules Local Modules Third-Party Modules

How does Node.js support multi-processor platforms, and does it fully utilize all processor resources?

Since Node.js is by default a single thread application, it will run on a single processor core and will not take full advantage of multiple core resources. However, Node.js provides support for deployment on multiple-core systems, to take greater advantage of the hardware. The Cluster module is one of the core Node.js modules and it allows running multiple Node.js worker processes that will share the same port.

What are streams in Node.js?

Streams are methods used to handle reading/writing files, network communication, or end-to-end information exchange. These methods are used to process large data by reading data piece by piece, process its content, especially for data files larger than free memory space. Streams help to read and write data as: Readable: Data can be read from the stream using fs.createReadStream() command. Writable: Data can be written into the stream using fs.createWriteStream() command. Duplex: Readable and writable streams are duplex and use net.Socket command. Transform: Duplex streams converting read and written data as zlib.createDeflate().

What is the difference between synchronous and asynchronous functions?

Synchronous functions are mainly used for I/O operations. They are instantaneous in providing a response to the data movement in the server and keep up with the data as per the requirement. If there are no responses, then the API will throw an error. On the other hand, asynchronous functions, as the name suggests, work on the basis of not being synchronous. Here, HTTP requests when pushed will not wait for a response to begin. Responses to any previous requests will be continuous even if the server has already got the response. Next among the Node JS questions, you have to learn about the control flow function.

What is V8?

The V8 library provides Node.js with a JavaScript engine (a program that converts Javascript code into lower level or machine code that microprocessors can understand), which Node.js controls via the V8 C++ API. V8 is maintained by Google, for use in Chrome. The Chrome V8 engine : - The V8 engine is written in C++ and used in Chrome and Nodejs. - It implements ECMAScript as specified in ECMA-262. - The V8 engine can run standalone we can embed it with our own C++ program.

Explain what is wrong with async/await use in the forEach loop

The code doesn't do what you expect it to do. It just fires off multiple asynchronous calls, but the function does immediately return after that. TO GET IT TO WORK: If you want to read the files in sequence, you cannot use forEach indeed. Just use a modern for ... of loop instead, in which await will work as expected:

What is the control flow function?

The control flow function is a common code snippet, which executes whenever there are any asynchronous function calls made, and they are used to evaluate the order in which these functions are executed in Node.js.

What is the order of execution in control flow statements?

The following is the order in which control flow statements are used to process function calls: Handling execution and queue Data collection and storage Concurrency handling and limiting Execution of the next piece of code

Explain the order of Event Listeners execution in Node.js

The listeners are executed in the order the listeners are created for an event emitter.

Explain the difference between local and global npm packages installation

The main difference between local and global packages is this: local packages are installed in the directory where you run npm install <package-name>, and they are put in the node_modules folder under this directory global packages are all put in a single place in your system (exactly where depends on your setup), regardless of where you run npm install -g <package-name> In general, all packages should be installed locally. This makes sure you can have dozens of applications in your computer, all running a different version of each package if needed. Updating a global package would make all your projects use the new release, and as you can imagine this might cause nightmares in terms of maintenance, as some packages might break compatibility with further dependencies, and so on.

Are you familiar with differences between Node.js modules and ES6 modules?

The modules used in Node.js follow a module specification known as the CommonJS specification. The recent updates to the JavaScript programming language, in the form of ES6, specify changes to the language, adding things like new class syntax and a module system. This module system is different from Node.js modules. To import ES6 module, we'd use the ES6 import functionality. Now ES6 modules are incompatible with Node.js modules. This has to do with the way modules are loaded differently between the two formats. If you use a compiler like Babel, you can mix and match module formats.

FIX THIS for (var i = 0; i < 5; i++) { setTimeout(function () { console.log(i); }, i); } outputs 5,5,5,5,5

The reason this happens is because each timeout is created and then i is incremented. Then when the callback is called, it looks for the value of i and it is 5. The solution is to create a closure so that the current value of i is stored. For example: for (var i = 0; i < 5; i++) { (function(i) { setTimeout(function () { console.log(i); }, i); })(i); }

14. What are the input arguments for an asynchronous queue?

There are two main arguments that an asynchronous queue uses. They are: Concurrency value Task function

What are the different API functions supported by Node.js?

There are two types of API functions. They are as follows: Synchronous APIs: Used for non-blocking functions Asynchronous APIs: Used for blocking functions

Describe Timer() module methods in Node.js

Timer module has various methods like setTimeout(), setImmediate(), setInterval().

Use of URL module in Node.js

Uniform Resource Identifier (URL) is composed of various portions such as protocol, host and port, filepath, and filename. URL module of Node.js help split web address (URL) into sections that user can understand.

What is an event loop in Node.js?

When running an application, callbacks are entities that have to be handled. In the case of Node.js, event loops are used for this purpose. Since Node.js supports the non-blocking send, this is a very important feature to have. The working of an event loop begins with the occurrence of a callback wherever an event begins. This is usually run by a specific listener. Node.js will keep executing the code after the functions have been called, without expecting the output prior to the beginning. Once, all of the code is executed, outputs are obtained and the callback function is executed. This works in the form of a continuous loop, hence the name event loop.

Why we always require modules at the top of a file? Can we require modules inside of functions?

Yes, we can but we shall never do it. Node.js always runs require synchronously. If you require an external module from within functions your module will be synchronously loaded when those functions run and this can cause two problems: If that module is only needed in one route handler function it might take some time for the module to load synchronously. As a result, several users would be unable to get any access to your server and requests will queue up. If the module you require causes an error and crashes the server you may not know about the error.

What is the difference between fs.readFile() and fs.createReadStream() in Node.js?

fs.readFile() - Using file system module, fs.readFile loads entire file you want to manage into the memory, reads entire file before it is sent to client - fs.readFile() is memory intensive for high content data files fs.createReadStream() fs.CreateReadStream reads entire file in chunks of pieces that we specify fs.createReadStream() is effective in memory efficient way of handling data intensive processing of large data files.

What is libuv?

libuv is a C library that is used to abstract non-blocking I/O operations to a consistent interface across all supported platforms. It provides mechanisms to handle file system, DNS, network, child processes, pipes, signal handling, polling and streaming. It also includes a thread pool for offloading work for some things that can't be done asynchronously at the operating system level.

Explain package.json file of Node.js

package.json is a JSON file present at the root directory of Node.js and contains metadata about projects like description, version, distribution, license, configuration related to end-user of project, and npm. This file identifies the project and handles the dependencies, provides information about project metadata values to npm.

What is the difference between setImmediate() and setTimeout()?

setTimeout() is executed after process.nextTick() i.e. after current code executed and before any I/O events. setTimeout() arranges script run after minimum threshold of milliseconds has completed. setImmediate() is executed after setTimeout(), with callback placed in check queue of next cycle of event loop. This command is processed on the check handler phase of the event loop. setImmediate() executes script after current poll phase completes.