An introduction for nodejs interview questions for experienced
Nodejs jobs demand increased rapidly in the last few years and it is expected that this demand will be higher in the next few years also. In this post, I provide a list of nodejs interview questions and answers for experienced developers. The post is interested to give you answers to the most common nodejs interview-asked questions. The post tries to illustrate nodejs interview questions for experienced professionals and their answers. The nodejs interview questions for experienced can be a good start for professional developers in preparing the interview with software companies.
What is javascript?
JavaScript is without doubt the most widely used language on the planet. It powers the modern web, it powers smartphones, and it even runs on your TV. Its popularity is not limited to the client, however; Nodejs, which allows developers to write server-side applications in Javascript, is also growing like wildfire.
What is the nodejs
Node.js is an open-source, cross-platform JavaScript run-time environment that executes JavaScript code server-side. It is a platform built on Chrome’s V8 JavaScript engine. Node.js allows software developers to write server-side applications using JavaScript, and its asynchronous I/O and event-driven programming capabilities make it easy to build scalable, non-blocking network applications that run across distributed devices.
What is the use of REPL in Node.js?
REPL stands for Read-Eval-Print-Loop. REPL provides the user with a virtual javascript test environment. To launch it, a command called ‘node’ is used. After this, JavaScript commands can be typed directly into the command line.
If Node.js is single-threaded then how does it handle concurrency?
Node provides a single thread to programmers so that code can be written easily and without a bottleneck. Node internally uses multiple POSIX threads for various I/O operations such as File, DNS, Network calls, etc. When Node gets an I/O request it creates or uses a thread to perform that I/O operation and once the operation is done, it pushes the result to the event queue. On each such event, the event loop runs and checks the queue and if the execution stack of Node is empty then it adds the queue result to the execution stack.
What is a memory leak and how to avoid the memory leak in nodejs
At first, let’s understand what means by the memory leak, The memory leak means blocks (places) in memory that is no longer used by the application, the problem is that such unused blocks may grow with time and can cause an application crash. Node.js, like all javascript-based applications, suffers from memory leakage. If a script is not closed properly any object reference created in it will stay in memory until the node application is restarted. The problem with this is that the actual garbage collection occurs very rarely, once per minute.
Memory leak causes
Usage of global variables
Global variables in Node.js are the variables referenced by the root node, which is global. It’s the equivalent of the window for JavaScript running in the browser. Global variables are never garbage collected throughout the lifetime of an app. They occupy memory as long as the app is running. Here’s the kicker: this applies to any object referenced by a global variable, and all their children, as well. Having a large graph of objects referenced from the root can lead to a memory leak in Node.js applications.
Multiple/shared reference
When an object is referenced from multiple other objects.
Timers and Events
as observers, event emitters, event listeners, setTimeout, setInterval, setImmediate
How do you avoid memory leaks?
- Use strict mode
- Avoid using global variables
- Avoid the store the big size objects inside the global scope
- Use Stack Memory and heap memory Effectively
- Use node-inspector
- Use dev tools
- Use monitoring tools
Describe the nodejs event loop
The event loop is an abstraction that allows JavaScript code to be run in a non-blocking way. The event loop in Node.js is the fundamental synchronization primitive upon which asynchronous programming is built. The Node.js event loop runs a single thread and uses mechanisms such as timers and callbacks to achieve concurrency. In this way, it is very similar to Java’s threading model, but it differs in that instead of having a thread for each request it has a thread for each logical request.
Describe the nodejs EventEmitter
EventEmitter is a class that holds all the objects that can emit events. Whenever an object from the EventEmitter class throws an event, all attached functions are called upon synchronously
Describe the nodejs callback function
A callback is a function called after a given task. This prevents any blocking and enables other code to run in the meantime. In the last section, we will now cover some of the advanced-level Node.js interview questions.
What is piping in Node.js?
Piping is a mechanism used to connect the output of one stream to another stream. It is normally used to retrieve data from one stream and pass output to another stream.
What is the buffer class in Node.js?
Buffer class stores raw data similar to an array of integers but corresponds to a raw memory allocation outside the V8 heap. Buffer class is used because pure JavaScript is not compatible with binary data
What are the pros of using promises in nodejs? what is its alternative?
Pros
- The control flow of asynchronous logic is more specified and structured.
- The coupling is low.
- We’ve built-in error handling.
- Improved readability.
Cons
- It may cause a memory leak
Alternatives
- use async/await
Count a set of the most common libraries in nodejs
- Express
- Gulp
- Request
- PM2
- Parser
- Cors
- Passport
- Axios
- Multer
- Dotenv
- Sequelize
you can check more about nodejs libraries here
What are global objects in Node.js?
Global variables are seen in all the contexts of the application. To make the variable global you just define it with global and then “.”. For example, in any of the js files if you write
global.myNumber = 5;
The “myNumber” variable will be global and can be accessed via all the applications.
Note: global variables can cause memory leaks, so use them carefully.
What is middleware? what is the middleware usage in nodejs?
The middleware in node. js is a function that will have all the access for requesting an object, responding to an object, and moving to the next middleware function in the application request-response cycle. For example, you can use SQL injection middleware which can be used to check if any request has vulnerabilities of SQL injection.
function logOriginalUrl (req, res, next) {
console.log('Request URL:', req.originalUrl)
next()
}
function checkSqlInjection(req, res, next) {
let requestBody = req.body
// Check sql injection stuff
next()
}
const middleWares = [logOriginalUrl, checkSqlInjection]
app.get('/customer/:id', middleWares , (req, res, next) => {
res.send('customer data')
})
How can you enhance the nodejs application performance?
You can enhance the nodejs application performance using the cluster mode. The cluster mode is used to start up many nodejs instances of the event loop at the same time. When we start using a cluster in a nodejs app behind the scene multiple node.js processes are created but there is also a parent process called the cluster manager which is responsible for monitoring the health of the individual instances of our application.
What are some commonly used timing features of Node.js?
- setTimeout/clearTimeout – This is used to implement delays in code execution.
- setInterval/clearInterval – This is used to run a code block multiple times.
- setImmediate/clearImmediate – Any function passed as the setImmediate() argument is a callback that’s executed in the next iteration of the event loop.
- process.nextTick – Both setImmediate and process.nextTick appear to be doing the same thing; however, you may prefer one over the other depending on your callback’s urgency.
What is the difference between setImmediate() and setTimeout()?
The setImmediate() function is meant to execute a single script once the current event loop is complete. The setTimeout() function is used to hold a script and schedule it to be run after a certain time threshold is over. The order of execution will solely depend on the context in which the functions are called. If called from the main module, the timing will be based on the performance of the process.
What is the difference between process.nextTick() and setImmediate()?
The distinction between method and product. This is accomplished through the use of nextTick() and setImmediate(). next Tick() postpones the execution of action until the next pass around the event loop, or it simply calls the callback function once the event loop’s current execution is complete, whereas setImmediate() executes a callback on the next cycle of the event loop and returns control to the event loop for any I/O operations.
What are the differences between worker thread and cluster?
Cluster
- There is one process on each CPU with an IPC to communicate.
- In case we want to have multiple servers accepting HTTP requests via a single port, clusters can be helpful.
- The processes are spawned in each CPU and thus will have separate memory and node instance which further will lead to memory issues.
Worker thread
- There is only one process in total with multiple threads.
- Each thread has one Node instance (one event loop, one JS engine) with most of the APIs accessible.
- Shares memory with other threads (e.g. SharedArrayBuffer)
- This can be used for CPU-intensive tasks like processing data or accessing the file system since NodeJS is single-threaded, synchronous tasks can be made more efficient by leveraging the worker’s threads.
Conclusion
The nodejs jobs demand increases, so to crack the nodejs interview you have to put some effort. In this post, I added some common questions in the nodejs interviews providing the answers to every question. The nodejs interview questions for experienced can be a good start for professional developers in preparing the interview
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