Node JS Interview Question Part 3

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.

The cluster module helps to spawn new processes on the operating system. Each process works independently, so you cannot use shared state between child processes. Each process communicates with the main process by IPC and pass server handles back and forth.

Cluster supports two types of load distribution:

  • The main process listens on a port, accepts new connection and assigns it to a child process in a round robin fashion.
  • The main process assigns the port to a child process and child process itself listen the port.

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

The first argument to any callback handler is an optional error object

function callback(err, results) {
    // usually we'll check for the error before handling results
    if(err) {
        // handle error somehow and return
    }
    // no error, perform standard callback handling
}

How Node.js read the content of a file?

The “normal” way in Node.js is probably to read in the content of a file in a non-blocking, asynchronous way. That is, to tell Node to read in the file, and then to get a callback when the file-reading has been finished. That would allow us to hand several requests in parallel.

Common use for the File System module:

  • Read files
  • Create files
  • Update files
  • Delete files
  • Rename files

Read Files
index.html

<html>
<body>
  <h1>My Header</h1>
  <p>My paragraph.</p>
</body>
</html>
// read_file.js

var http = require('http');
var fs = require('fs');
http.createServer(function (req, res) {
  fs.readFile('index.html', function(err, data) {
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.write(data);
    res.end();
  });
}).listen(8080);

Initiate read_file.js:

node read_file.js

Node JS Interview Question

What is JIT and how is it related to Node.js?

Node.js has depended on the V8 JavaScript engine to provide code execution in the language. The V8 is a JavaScript engine built at the google development center, in Germany. It is open source and written in C++. It is used for both client side (Google Chrome) and server side (node.js) JavaScript applications. A central piece of the V8 engine that allows it to execute JavaScript at high speed is the JIT (Just In Time) compiler. This is a dynamic compiler that can optimize code during runtime. When V8 was first built the JIT Compiler was dubbed FullCodegen. Then, the V8 team implemented Crankshaft, which included many performance optimizations that FullCodegen did not implement.

The V8 was first designed to increase the performance of the JavaScript execution inside web browsers. In order to obtain speed, V8 translates JavaScript code into more efficient machine code instead of using an interpreter. It compiles JavaScript code into machine code at execution by implementing a JIT (Just-In-Time) compiler like a lot of modern JavaScript engines such as Spider Monkey or Rhino (Mozilla) are doing. The main difference with V8 is that it doesn’t produce bytecode or any intermediate code.

What is chrome v8 engine?

V8 is the name of the JavaScript engine that powers Google Chrome. It’s the thing that takes our JavaScript and executes it while browsing with Chrome. V8 provides the runtime environment in which JavaScript executes. The DOM, and the other Web Platform APIs are provided by the browser.

V8 is Google’s open source high-performance JavaScript and WebAssembly engine, written in C++. It is used in Chrome and in Node.js, among others. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors. V8 can run standalone, or can be embedded into any C++ application.

What is difference between put and patch?

PUT and PATCH are HTTP verbs and they both relate to updating a resource. The main difference between PUT and PATCH requests are in the way the server processes the enclosed entity to modify the resource identified by the Request-URI.

In a PUT request, the enclosed entity is considered to be a modified version of the resource stored on the origin server, and the client is requesting that the stored version be replaced.

With PATCH, however, the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version.

Also, another difference is that when you want to update a resource with PUT request, you have to send the full payload as the request whereas with PATCH, you only send the parameters which you want to update.

The most commonly used HTTP verbs POST, GET, PUT, DELETE are similar to CRUD (Create, Read, Update and Delete) operations in database. We specify these HTTP verbs in the capital case. So, the below is the comparison between them.

  • POST – create
  • GET – read
  • PUT – update
  • DELETE – delete

PATCH: Submits a partial modification to a resource. If you only need to update one field for the resource, you may want to use the PATCH method.

Advance Node JS Interview Question

List types of Http requests supported by Node.js?

The HTTP core module is a key module to Node.js networking.

const http = require('http')

http.METHODS

require('http').METHODS
[ 'ACL',
  'BIND',
  'CHECKOUT',
  'CONNECT',
  'COPY',
  'DELETE',
  'GET',
  'HEAD',
  'LINK',
  'LOCK',
  'M-SEARCH',
  'MERGE',
  'MKACTIVITY',
  'MKCALENDAR',
  'MKCOL',
  'MOVE',
  'NOTIFY',
  'OPTIONS',
  'PATCH',
  'POST',
  'PROPFIND',
  'PROPPATCH',
  'PURGE',
  'PUT',
  'REBIND',
  'REPORT',
  'SEARCH',
  'SUBSCRIBE',
  'TRACE',
  'UNBIND',
  'UNLINK',
  'UNLOCK',
  'UNSUBSCRIBE' ]

http.STATUS_CODES

require('http').STATUS_CODES
{ '100': 'Continue',
'101': 'Switching Protocols',
'102': 'Processing',
'200': 'OK',
'201': 'Created',
'202': 'Accepted',
'203': 'Non-Authoritative Information',
'204': 'No Content',
'205': 'Reset Content',
'206': 'Partial Content',
'207': 'Multi-Status',
'208': 'Already Reported',
'226': 'IM Used',
'300': 'Multiple Choices',
'301': 'Moved Permanently',
'302': 'Found',
'303': 'See Other',
'304': 'Not Modified',
'305': 'Use Proxy',
'307': 'Temporary Redirect',
'308': 'Permanent Redirect',
'400': 'Bad Request',
'401': 'Unauthorized',
'402': 'Payment Required',
'403': 'Forbidden',
'404': 'Not Found',
'405': 'Method Not Allowed',
'406': 'Not Acceptable',
'407': 'Proxy Authentication Required',
'408': 'Request Timeout',
'409': 'Conflict',
'410': 'Gone',
'411': 'Length Required',
'412': 'Precondition Failed',
'413': 'Payload Too Large',
'414': 'URI Too Long',
'415': 'Unsupported Media Type',
'416': 'Range Not Satisfiable',
'417': 'Expectation Failed',
'418': 'I\'m a teapot',
'421': 'Misdirected Request',
'422': 'Unprocessable Entity',
'423': 'Locked',
'424': 'Failed Dependency',
'425': 'Unordered Collection',
'426': 'Upgrade Required',
'428': 'Precondition Required',
'429': 'Too Many Requests',
'431': 'Request Header Fields Too Large',
'451': 'Unavailable For Legal Reasons',
'500': 'Internal Server Error',
'501': 'Not Implemented',
'502': 'Bad Gateway',
'503': 'Service Unavailable',
'504': 'Gateway Timeout',
'505': 'HTTP Version Not Supported',
'506': 'Variant Also Negotiates',
'507': 'Insufficient Storage',
'508': 'Loop Detected',
'509': 'Bandwidth Limit Exceeded',
'510': 'Not Extended',
'511': 'Network Authentication Required' }

Making HTTP Requests

const request = require('request');

request('https://nodejs.org/', function(err, res, body) {
    console.log(body);
});

The first argument to request can either be a URL string, or an object of options. Here are some of the more common options you’ll encounter in your applications:

  • url: The destination URL of the HTTP request
  • method: The HTTP method to be used (GET, POST, DELETE, etc)
  • headers: An object of HTTP headers (key-value) to be set in the request
  • form: An object containing key-value form data
const request = require('request');

const options = {
    url: 'https://nodejs.org/file.json',
    method: 'GET',
    headers: {
        'Accept': 'application/json',
        'Accept-Charset': 'utf-8',
        'User-Agent': 'my-reddit-client'
    }
};

request(options, function(err, res, body) {
    let json = JSON.parse(body);
    console.log(json);
});

Using the options object, this request uses the GET method to retrieve JSON data directly from Reddit, which is returned as a string in the body field. From here, you can use JSON.parse and use the data as a normal JavaScript object.

Why to use Express.js?

Express JS is a prebuilt NodeJS framework that can help you in creating server-side web applications faster and smarter. Simplicity, minimalism, flexibility, scalability are some of its characteristics and since it is made in NodeJS itself, it inherited its performance as well.

Express 3.x is a light-weight web application framework to help organize your web application into an MVC architecture on the server side. You can then use a database like MongoDB with Mongoose (for modeling) to provide a backend for your Node.js application. Express.js basically helps you manage everything, from routes, to handling requests and views.

It has become the standard server framework for node.js. Express is the backend part of something known as the MEAN stack. The MEAN is a free and open-source JavaScript software stack for building dynamic web sites and web applications which has the following components;

  1. MongoDB – The standard NoSQL database
  2. Express.js – The default web applications framework
  3. Angular.js – The JavaScript MVC framework used for web applications
  4. Node.js – Framework used for scalable server-side and networking applications.

The Express.js framework makes it very easy to develop an application which can be used to handle multiple types of requests like the GET, PUT, and POST and DELETE requests.

using Express

var express=require('express');
var app=express();
app.get('/',function(req,res) {
  res.send('Hello World!');
});
var server=app.listen(3000,function() {});

Write the steps for setting up an Express JS application?

1. Install Express Generator

C:\node>npm install -g express-generator

2. Create an Express Project

C:\node>express --view="ejs" nodetest1

3. Edit Dependencies

MAKE SURE TO CD INTO YOUR nodetest FOLDER. OK, now we have some basic structure in there, but we’re not quite done. You’ll note that the express-generator routine created a file called package.json in your nodetest1 directory. Open this up in a text editor and it’ll look like this:

// C:\node\nodetest1\package.json
{
  "name": "nodetest1",
  "version": "0.0.0",
  "private": true,
  "scripts": {
    "start": "node ./bin/www"
  },
  "dependencies": {
    "cookie-parser": "~1.4.3",
    "debug": "~2.6.9",
    "ejs": "~2.5.7",
    "express": "~4.16.0",
    "http-errors": "~1.6.2",
    "morgan": "~1.9.0"
  }
}

This is a basic JSON file describing our app and its dependencies. We need to add a few things to it. Specifically, calls for MongoDB and Monk.

C:\node\nodetest1>npm install --save monk@^6.0.6 mongodb@^3.1.13

4. Install Dependencies

C:\node\nodetest1>npm install
C:\node\nodetest1>npm start

Node Console

> nodetest1@0.0.0 start C:\node\nodetest1
> node ./bin/www

Node JS Interview Question

Since node is a single threaded process, how to make use of all CPUs?

Node.js is a single threaded language which in background uses multiple threads to execute asynchronous code. Node.js is non-blocking which means that all functions ( callbacks ) are delegated to the event loop and they are ( or can be ) executed by different threads. That is handled by Node.js run-time.

  • Node.js does support forking multiple processes ( which are executed on different cores ).
  • It is important to know that state is not shared between master and forked process.
  • We can pass messages to forked process ( which is different script ) and to master process from forked process with function send.

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load. The cluster module allows easy creation of child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Running Node.js will now share port 8000 between the workers:

$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

What does emitter do and what is dispatcher?

Node.js core API is based on asynchronous event-driven architecture in which certain kind of objects called emitters periodically emit events that cause listener objects to be called.

All objects that emit events are members of EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. All values returned by the called listeners are ignored and will be discarded.

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this);
  // Prints:
  //   Technoetics Club MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined }
});
myEmitter.emit('event','Technoetics', 'Club');

Here we create a myEmitter object and emit event at the end which triggers the callback function and we are able to get the desired output.

By default, all listeners attached to a particular event object are called by the EventListener object synchronously in the order in which they are registered or attached to the event object.

Dispatcher

The Dispatcher has functionality not provided nor expected in EventEmitter, the most notable being waitFor, which allows a store to ensure that another store has been updated in response to an action before it proceeds.

Pattern-wise, the Dispatcher is also a singleton, whereas EventEmitter is an API that you might object-assign onto multiple stores.

How to kill child processes that spawn their own child processes in Node.js?

If a child process in Node.js spawn their own child processes, kill() method will not kill the child process’s own child processes. For example, if I start a process that starts it’s own child processes via child_process module, killing that child process will not make my program to quit.

var spawn = require('child_process').spawn;
var child = spawn('my-command');

child.kill();

The program above will not quit if my-command spins up some more processes.

PID range hack

We can start child processes with {detached: true} option so those processes will not be attached to main process but they will go to a new group of processes. Then using process.kill(-pid) method on main process we can kill all processes that are in the same group of a child process with the same pid group. In my case, I only have one processes in this group.

var spawn = require('child_process').spawn;
var child = spawn('my-command', {detached: true});

process.kill(-child.pid);

Please note – before pid. This converts a pid to a group of pids for process kill() method.

Advance Node JS Interview Question

What do you understand by Reactor Pattern in Node.js?

Reactor Pattern is an idea of non-blocking I/O operations in Node.js. This pattern provides a handler(in case of Node.js, a callback function) that is associated with each I/O operation. When an I/O request is generated, it is submitted to a demultiplexer.

This demultiplexer is a notification interface that is used to handle concurrency in non-blocking I/O mode and collects every request in form of an event and queues each event in a queue. Thus, the demultiplexer provides the Event Queue, which we often hear. When a request is collected by the demultiplexer, it returns the control back to the system and does not blocks the I/O. At the same time, there is an Event Loop which iterates over the items in the Event Queue. Every event has a callback function associated with it, and that callback function is invoked when the Event Loop iterates.

The callback function further mostly have other callbacks associated within representing some asynchronous operations. These operations are inserted in the Event Queue by the demultiplexer and are ready to be processed once the Event Loop iterates over them. That is why calls to other operations must be asynchronous.

When all the items in the Event Queue are processed and there are no pending operations left, Node.js terminates the application automatically.

Test Pyramid
  1. The application generates a new I/O operation by submitting a request to the Event Demultiplexer. The application also specifies a handler, which will be invoked when the operation completes. Submitting a new request to the Event Demultiplexer is a non-blocking call and it immediately returns the control back to the application.
  2. When a set of I/O operations completes, the Event Demultiplexer pushes the new events into the Event Queue.
  3. At this point, the Event Loop iterates over the items of the Event Queue.
  4. For each event, the associated handler is invoked.
  5. The handler, which is part of the application code, will give back the control to the Event Loop when its execution completes (5a). However, new asynchronous operations might be requested during the execution of the handler (5b), causing new operations to be inserted in the Event Demultiplexer (1), before the control is given back to the Event Loop.
  6. When all the items in the Event Queue are processed, the loop will block again on the Event Demultiplexer which will then trigger another cycle.

What are the key features of Node.js?

  • Asynchronous event driven IO helps concurrent request handling – All APIs of Node.js are asynchronous. This feature means that if a Node receives a request for some Input/output operation, it will execute that operation in the background and continue with the processing of other requests. Thus it will not wait for the response from the previous requests.
  • Fast in Code execution – Node.js uses the V8 JavaScript Runtime engine, the one which is used by Google Chrome. Node has a wrapper over the JavaScript engine which makes the runtime engine much faster and hence processing of requests within Node.js also become faster.
  • Single Threaded but Highly Scalable – Node.js uses a single thread model for event looping. The response from these events may or may not reach the server immediately. However, this does not block other operations. Thus making Node.js highly scalable. Traditional servers create limited threads to handle requests while Node.js creates a single thread that provides service to much larger numbers of such requests.
  • Node.js library uses JavaScript – This is another important aspect of Node.js from the developer’s point of view. The majority of developers are already well-versed in JavaScript. Hence, development in Node.js becomes easier for a developer who knows JavaScript.
  • There is an Active and vibrant community for the Node.js framework – The active community always keeps the framework updated with the latest trends in the web development.
  • No Buffering – Node.js applications never buffer any data. They simply output the data in chunks.

What are global in Node.js?

There are three keywords in Node.js which constitute as Global. These are Global, Process, and Buffer.

1. Global

The Global keyword represents the global namespace object. It acts as a container for all other global objects. If we type console.log(global), it will print out all of them.

An important point to note about the global objects is that not all of them are in the global scope, some of them fall in the module scope. So, it is wise to declare them without using the var keyword or add them to Global object.

Variables declared using the var keyword become local to the module whereas those declared without it get subscribed to the global object.

2. Process

It is also one of the global objects but includes additional functionality to turn a synchronous function into an async callback. There is no boundation to access it from anywhere in the code. It is the instance of the EventEmitter class. And each node application object is an instance of the Process object.

It primarily gives back the information about the application or the environment.

  • <process.execPath> – to get the execution path of the Node app.
  • <process.Version> – to get the Node version currently running.
  • <process.platform> – to get the server platform.

Some of the other useful Process methods are as follows.

  • <process.memoryUsage> – To know the memory used by Node application.
  • <process.NextTick> – To attach a callback function that will get called during the next loop. It can cause a delay in executing a function.

3. Buffer

The Buffer is a class in Node.js to handle binary data. It is similar to a list of integers but stores as a raw memory outside the V8 heap.

We can convert JavaScript string objects into Buffers. But it requires mentioning the encoding type explicitly.

  • <ascii> – Specifies 7-bit ASCII data.
  • <utf8> – Represents multibyte encoded Unicode char set.
  • <utf16le> – Indicates 2 or 4 bytes, little endian encoded Unicode chars.
  • <base64> – Used for Base64 string encoding.
  • <hex> – Encodes each byte as two hexadecimal chars.

Here is the syntax to use the Buffer class.

var buffer = new Buffer(string, [encoding]);

The above command will allocate a new buffer holding the string with utf8 as the default encoding. However, if you like to write a string to an existing buffer object, then use the following line of code.

buffer.write(string)

This class also offers other methods like readInt8 and writeUInt8 that allows read/write from various types of data to the buffer.

Node JS Interview Question

What is chaining process in Node.js?

It is an approach to connect the output of one stream to the input of another stream, thus creating a chain of multiple stream operations.

What is a control flow function? What are the steps does it execute?

It is a generic piece of code which runs in between several asynchronous function calls is known as control flow function.

It executes the following steps.

  • Control the order of execution.
  • Collect data.
  • Limit concurrency.
  • Call the next step in the program.

When to use Node.js and when not to use it?

When to use Node.js

It is ideal to use Node.js for developing streaming or event-based real-time applications that require less CPU usage such as.

  • Chat applications.
  • Game servers — Node.js is good for fast and high-performance servers, that face the need to handle thousands of user requests simultaneously.
  • Good For A Collaborative Environment — It is suitable for environments where multiple people work together. For example, they post their documents, modify them by doing check-out and check-in of these documents. Node.js supports such situations by creating an event loop for every change made to the document. The “Event loop” feature of Node.js enables it to handle multiple events simultaneously without getting blocked.
  • Advertisement Servers — Here again, we have servers that handle thousands of request for downloading advertisements from a central host. And Node.js is an ideal solution to handle such tasks.
  • Streaming Servers — Another ideal scenario to use Node.js is for multimedia streaming servers where clients fire request’s towards the server to download different multimedia contents from it.

To summarize, it’s good to use Node.js, when you need high levels of concurrency but less amount of dedicated CPU time.

Last but not the least, since Node.js uses JavaScript internally, so it fits best for building client-side applications that also use JavaScript.

When to not use Node.js

However, we can use Node.js for a variety of applications. But it is a single threaded framework, so we should not use it for cases where the application requires long processing time. If the server is doing some calculation, it won’t be able to process any other requests. Hence, Node.js is best when processing needs less dedicated CPU time.

Advance Node JS Interview Question

Is Node.js entirely based on a single-thread?

Yes, it is true that Node.js processes all requests on a single thread. But it is just a part of the theory behind Node.js design. In fact, more than the single thread mechanism, it makes use of events and callbacks to handle a large no. of requests asynchronously.

Moreover, Node.js has an optimized design which utilizes both JavaScript and C++ to guarantee maximum performance. JavaScript executes at the server-side by Google Chrome v8 engine. And the C++ lib UV library takes care of the non-sequential I/O via background workers.

To explain it practically, let’s assume there are 100s of requests lined up in Node.js queue. As per design, the main thread of Node.js event loop will receive all of them and forwards to background workers for execution. Once the workers finish processing requests, the registered callbacks get notified on event loop thread to pass the result back to the user.

How to make post request in Node.js?

Following code snippet can be used to make a Post Request in Node.js.

var request = require('request');
    request.post('http://www.example.com/action', { form: { key: 'value' } },
    function (error, response, body) {
        if (!error && response.statusCode == 200) {
            console.log(body)
        }
    });

Can you create http server in Node.js, explain the code used for it?

Yes, we can create HTTP Server in Node.js. We can use the <http-server> command to do so.

Following is the sample code.

var http = require('http');
var requestListener = function (request, response) {
    response.writeHead(200, {'Content-Type': 'text/plain'});
    response.end('Welcome Viewers\n');
}
var server = http.createServer(requestListener);
server.listen(4200); // The port where you want to start with.

Node JS Interview Question

How to load html in Node.js?

To load HTML in Node.js we have to change the “Content-type” in the HTML code from text/plain to text/html.

fs.readFile(filename, "binary", function(err, file) {
    if(err) { 
        response.writeHead(500, {"Content-Type": "text/plain"});
        response.write(err + "\n");
        response.end();
        return;
    }

response.writeHead(200);
response.write(file, "binary");
response.end();
});

Now we will modify this code to load an HTML page instead of plain text.

fs.readFile(filename, "binary", function(err, file) {
    if(err) { 
        response.writeHead(500, {"Content-Type": "text/html"});
        response.write(err + "\n");
        response.end();
        return;
    }

response.writeHead(200, {"Content-Type": "text/html"});
response.write(file);
response.end();
});

How can you listen on port 80 with Node?

Instead of running on port 80 we can redirect port 80 to your application’s port (>1024) using

iptables -t nat -I PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 3000

What is the difference between operational and programmer errors?

Operation errors are not bugs, but problems with the system, like request timeout or hardware failure. On the other hand programmer errors are actual bugs.

Advance Node JS Interview Question

Why npm shrink-wrap is useful?

NPM shrink-wrap lets you lock down the ver­sions of installed pack­ages and their descen­dant pack­ages. It helps you use same package versions on all environments (development, staging, production) and also improve download and installation speed. Having same versions of packages on all environments can help you test systems and deploy with confidence. If all tests pass on one machine, you can be sure that it will pass on all other because you know that you use same code!

npm shrinkwrap

It should create new npm-shrinkwrap.json file with information about all packages you use.

What is your favourite HTTP framework and why?

Express.js

Express provides a thin layer on top of Node.js with web application features such as basic routing, middleware, template engine and static files serving, so the drastic I/O performance of Node.js doesn’t get compromised.

Express is a minimal, un-opinionated framework. it doesn’t apply any of the prevalent design patterns such as MVC, MVP, MVVM or whatever is trending out of the box. For fans of simplicity, this is a big plus among all other frameworks because you can build your application with your own preference and no unnecessary learning curve. This is especially advantageous when creating a new personal project with no historical burden, but as the project or developing team grows, lack of standardization may lead to extra work for project/code management, and worst case scenario it may lead to the inability to maintain.

Generator

Even though the framework is un-opinionated, it does have the generator that generates specific project folder structure. After installing express-generator npm package and creating application skeleton with generator command, an application folder with clear hierarchy will be created to help you organize images, front-end static JavaScript, stylesheet files and HTML template files.

npm install express-generator -g
express helloapp

Middleware

Middleware are basically just functions that have full access to both request and response objects.

var app = express();

app.use(cookieParser());
app.use(bodyParser());
app.use(logger());
app.use(authentication());

app.get('/', function (req, res) {
  // ...
});

app.listen(3000);

An Express application is essentially Node.js with a host of middleware functions, whether you want to customize your own middleware or take advantage of the built-in middleware’s of the framework, Express made the process natural and intuitive.

Template Engine

Template engines allow developer to embed backend variables into HTML files, and when requested the template file will be rendered to plain HTML format with the variables interpolated with their actual values. By default, the express-generator uses Pug (originally known as Jade) template engine, but other options like Mustache and EJS also work with Express seamlessly.

Database Integration

As a minimal framework, Express does not consider database integration as a required aspect within its package, thus it leans toward no specific database usage whatsoever. While adopting a particular data storage technology, be it MySQL, MongoDB, PostgreSQL, Redis, Elasticsearch or something else, it’s just a matter of installing the particular npm package as database driver. These third party database drivers do not conform to unified syntax when doing CRUD instructions, which makes switching databases a big hassle and error prone.

What are the Challenges with Node.js?

Challenges with Node.js Application Maintenance

Improper maintenance of an application can result in issues related to stability or flexibility, often leading to the app’s failure. If the code is not well-written or if developers use outdated tools, the performance can suffer, and users might experience more bugs and app crashes. On top of that, poor-quality code can hamper the app’s scaling capacity and the further development of the application. In the worst case scenario, it might become impossible to introduce new features without rewriting the codebase from scratch.

  1. Extensive stack
  2. Technical Debt
  3. Scalability challenges
  4. Poor documentation

How to Deal With Maintenance Problems

  1. Conduct code review
  2. Use microservices
  3. Improve code quality
  4. Test before new feature implementation
  5. Improve documentation
  6. Update the stack
  7. Dig into the roots

Node JS Interview Question

What is the difference between Node.js vs Ajax?

1. AJAX

AJAX stands for Asynchronous JavaScript and XML, it’s used to allow web pages (client-side) to update asynchronously by communicating with a web server and by exchanging data. This essentially means that applications can talk to a server in the background of the application. It uses some core components to function:

  1. The browser XMLHttpRequest object to request data from a server
  2. HTML/CSS to display or collect data
  3. DOM for dynamic display
  4. JSON/XML for interchanging the data
  5. Javascript to unify everything

2. Node.js

Node.js allows the developers to develop a web application in a single language called JavaScript for both client side and server side.

Unlike the other programming languages, Node.js has its cycle of the event in the form of language which is very beneficial for high-performance and scalable application development.

It is required for those web applications where traffic rate is very high. Node.js is an event based I/O language and its response time is very high rather than the other traditional languages. It is being used by the famous websites like Linked in, Twitter and Gmail.

The runtime environment of Node.js interprets JavaScript, which is very easy and simple to understand and code. Due to this reason, even the developers find it easy going which keeps them happy and relaxed. It is pertinent for real-time collaborative apps.

How Node.js overcomes the problem of blocking of I/O operations?

Node.js solves this problem by putting the event based model at its core, using an event loop instead of threads.

Node.js uses an event loop for this. An event loop is “an entity that handles and processes external events and converts them into callback invocations”. Whenever data is needed NodeJS registers a callback and sends the operation to this event loop. Whenever the data is available the callback is called.

Mention the steps by which you can async in Node.js?

ES 2017 introduced Asynchronous functions. Async functions are essentially a cleaner way to work with asynchronous code in JavaScript.

Async/Await

  • The newest way to write asynchronous code in JavaScript.
  • It is non blocking (just like promises and callbacks).
  • Async/Await was created to simplify the process of working with and writing chained promises.
  • Async functions return a Promise. If the function throws an error, the Promise will be rejected. If the function returns a value, the Promise will be resolved.

Syntax

// Normal Function
function add(x,y){
  return x + y;
}
// Async Function
async function add(x,y){
  return x + y;
}

Await

Async functions can make use of the await expression. This will pause the async function and wait for the Promise to resolve prior to moving on.

Example:

function doubleAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x * 2);
    }, 2000);
  });
}

async function addAsync(x) {
  const a = await doubleAfter2Seconds(10);
  const b = await doubleAfter2Seconds(20);
  const c = await doubleAfter2Seconds(30);
  return x + a + b + c;
}


addAsync(10).then((sum) => {
  console.log(sum);
});

Advance Node JS Interview Question

What is LTS releases of Node.js why should you care?

An LTS(Long Term Support) version of Node.js receives all the critical bug fixes, security updates and performance

LTS versions of Node.js are supported for at least 18 months and are indicated by even version numbers (e.g. 4, 6, 8). They’re best for production since the LTS release line is focused on stability and security, whereas the Current release line has a shorter lifespan and more frequent updates to the code. Changes to LTS versions are limited to bug fixes for stability, security updates, possible npm updates, documentation updates and certain performance improvements that can be demonstrated to not break existing applications.

Why should you separate Express ‘app’ and ‘server’?

Keeping the API declaration separated from the network related configuration (port, protocol, etc) allows testing the API in-process, without performing network calls, with all the benefits that it brings to the table: fast testing execution and getting coverage metrics of the code. It also allows deploying the same API under flexible and different network conditions. Bonus: better separation of concerns and cleaner code.

API declaration, should reside in app.js:

var app = express();
app.use(bodyParser.json());
app.use("/api/events", events.API);
app.use("/api/forms", forms);

Server network declaration, should reside in /bin/www:

var app = require('../app');
var http = require('http');

/**
 * Get port from environment and store in Express.
 */

var port = normalizePort(process.env.PORT || '3000');
app.set('port', port);

/**
 * Create HTTP server.
 */

var server = http.createServer(app);

What is the difference between process.nextTick() and setImmediate()?

The difference between process.nextTick() and setImmediate() is that process.nextTick() defers the execution of an action till the next pass around the event loop or it simply calls the callback function once the ongoing execution of the event loop is finished whereas setImmediate() executes a callback on the next cycle of the event loop and it gives back to the event loop for executing any I/O operations.

Node JS Interview Question

What are the difference between Events and Callbacks?

Node.js is a single-threaded application, but it support concurrency via the concept of event and callbacks. Every API of Node.js is asynchronous and being single-threaded, they use async function calls to maintain concurrency. Node thread keeps an event loop and whenever a task gets completed, it fires the corresponding event which signals the event-listener function to execute.

callback functions are called when an asynchronous function returns its result, whereas event handling works on the observer pattern. The functions that listen to events act as Observers. Whenever an event gets fired, its listener function starts executing. Node.js has multiple in-built events available through events module and EventEmitter class which are used to bind events and event-listeners

1. Callback: A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

Example: synchronous callback

function greeting(name) {
  alert('Hello ' + name);
}

function processUserInput(callback) {
  var name = prompt('Please enter your name.');
  callback(name);
}

processUserInput(greeting);

2. Events: Every action on a computer is an event. Node.js allows us to create and handle custom events easily by using events module. Event module includes EventEmitter class which can be used to raise and handle custom events.

Example:

var event = require('events');  
var eventEmitter = new event.EventEmitter();  
  
//  Add listener function for Sum event  
eventEmitter.on('Sum', function(num1, num2) {  
    console.log('Total: ' + (Number(num1) + Number(num2)));  
});  

//  Call Event.  
eventEmitter.emit('Sum', '10', '20');

Explain RESTful Web Services in Node.js?

REST stands for REpresentational State Transfer. REST is web standards based architecture and uses HTTP Protocol. It is an architectural style as well as an approach for communications purposes that is often used in various web services development. A REST Server simply provides access to resources and REST client accesses and modifies the resources using HTTP protocol.

HTTP methods

  • GET − Provides read-only access to a resource.
  • PUT − Updates an existing resource or creates a new resource.
  • DELETE − Removes a resource.
  • POST − Creates a new resource.
  • PATCH− Update/modify a resource

Principles of REST

  • Uniform Interface
  • Stateless
  • Cacheable
  • Client-Server
  • Layered System
  • Code on Demand (optional)

Example:

users.json

{
   "user1" : {
      "id": 1,
      "name" : "Ehsan Philip",
      "age" : 24
   },

   "user2" : {
      "id": 2,
      "name" : "Karim Jimenez",
      "age" : 22
   },

   "user3" : {
      "id": 3,
      "name" : "Giacomo Weir",
      "age" : 18
   }
}

List Users ( GET method)

Let’s implement our first RESTful API listUsers using the following code in a server.js file −

var express = require('express');
var app = express();
var fs = require("fs");

app.get('/listUsers', function (req, res) {
   fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
      console.log( data );
      res.end( data );
   });
})

var server = app.listen(3000, function () {
   var host = server.address().address
   var port = server.address().port
   console.log("App listening at http://%s:%s", host, port)
});

Add User ( POST method )

Following API will show you how to add new user in the list.

var express = require('express');
var app = express();
var fs = require("fs");

var user = {
   "user4" : {
      "id": 4,
      "name" : "Spencer Amos",
      "age" : 28
   }
}

app.post('/addUser', function (req, res) {
   // First read existing users.
   fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
      data = JSON.parse( data );
      data["user4"] = user["user4"];
      console.log( data );
      res.end( JSON.stringify(data));
   });
})

var server = app.listen(3000, function () {
   var host = server.address().address
   var port = server.address().port
   console.log("App listening at http://%s:%s", host, port)
})

Delete User

var express = require('express');
var app = express();
var fs = require("fs");

var id = 2;

app.delete('/deleteUser', function (req, res) {
   // First read existing users.
   fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
      data = JSON.parse( data );
      delete data["user" + 2];
      console.log( data );
      res.end( JSON.stringify(data));
   });
})

var server = app.listen(3000, function () {
   var host = server.address().address
   var port = server.address().port
   console.log("App listening at http://%s:%s", host, port)
})

How to handle file upload in Node.js?

  • express: Popular web framework built on top of Node.js, used for creating REST-API.
  • body-parser: Parse incoming request bodies in a middleware
  • multer: Middleware for handling multipart/form-data — file uploads

1. Installing the dependencies

npm install express body-parser multer --save

2. Package.json

{
  "name": "file_upload",
  "version": "0.0.1",
  "dependencies": {
    "express": "4.13.3",
    "multer": "1.1.0"
  },
  "devDependencies": {
    "should": "~7.1.0",
    "mocha": "~2.3.3",
    "supertest": "~1.1.0"
  }
}

3. Server.js

var express = require("express");
var bodyParser = require('body-parser');
var multer  = require('multer');
var app = express();

// for text/number data transfer between clientg and server
app.use(bodyParser());

var storage =   multer.diskStorage({
  destination: function (req, file, callback) {
    callback(null, './uploads');
  },
  filename: function (req, file, callback) {
    callback(null, file.fieldname + '-' + Date.now());
  }
});

var upload = multer({ storage : storage}).single('userPhoto');

app.get('/', function(req, res) {
      res.sendFile(__dirname + "/index.html");
});

// POST: upload for single file upload
app.post('/api/photo', function(req, res) {
    upload(req,res,function(err) {
        if(err) {
            return res.end("Error uploading file.");
        }
        res.end("File is uploaded");
    });
});

app.listen(3000, function(){
    console.log("Listening on port 3000");
});

4. index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Multer-File-Upload</title>
</head>
<body>
    <h1>MULTER File Upload | Single File Upload</h1> 

    <form id = "uploadForm"
         enctype = "multipart/form-data"
         action = "/api/photo"
         method = "post"
    >
      <input type="file" name="userPhoto" />
      <input type="submit" value="Upload Image" name="submit">
    </form>
</body>
</html>

Advance Node JS Interview Question

Explain the terms body-parser, cookie-parser, morgan, nodemon, pm2, serve-favicon, cors, dotenv, fs-extra, moment in Express JS?

a) body-parser

body-parser extract the entire body portion of an incoming request stream and exposes it on req.body. This body-parser module parses the JSON, buffer, string and URL encoded data submitted using HTTP POST request.

Example:

npm install express ejs body-parser
// server.js

var express = require('express')
var bodyParser = require('body-parser')

var app = express()

// create application/json parser
var jsonParser = bodyParser.json()

// create application/x-www-form-urlencoded parser
var urlencodedParser = bodyParser.urlencoded({ extended: false })

// POST /login gets urlencoded bodies
app.post('/login', urlencodedParser, function (req, res) {
  res.send('welcome, ' + req.body.username)
})

// POST /api/users gets JSON bodies
app.post('/api/users', jsonParser, function (req, res) {
  // create user in req.body
})

b) cookie-parser

A cookie is a piece of data that is sent to the client-side with a request and is stored on the client-side itself by the Web Browser the user is currently using.

The cookie-parser middleware’s cookieParser function takes a secret string or array of strings as the first argument and an options object as the second argument.

Installation

npm install cookie-parser

Example:

var express = require('express')
var cookieParser = require('cookie-parser')

var app = express()
app.use(cookieParser())

app.get('/', function (req, res) {
  // Cookies that have not been signed
  console.log('Cookies: ', req.cookies)

  // Cookies that have been signed
  console.log('Signed Cookies: ', req.signedCookies)
})

app.listen(3000)

c) morgan

HTTP request logger middleware for node.js.

Installation

npm install morgan

Example: write logs to a file

var express = require('express')
var fs = require('fs')
var morgan = require('morgan')
var path = require('path')

var app = express()

// create a write stream (in append mode)
var accessLogStream = fs.createWriteStream(path.join(__dirname, 'access.log'), { flags: 'a' })

// setup the logger
app.use(morgan('combined', { stream: accessLogStream }))

app.get('/', function (req, res) {
  res.send('hello, world!')
})

d) nodemon

Nodemon is a utility that will monitor for any changes in source and automatically restart your server.

Installation

npm install -g nodemon

Example:

{
  // ...
  "scripts": {
    "start": "nodemon server.js"
  },
  // ...
}

e) pm2

P(rocess) M(anager) 2 (pm2) is a production process manager for Node.js applications with a built-in load balancer. It allows to keep applications alive forever, to reload them without downtime and to facilitate common system admin tasks.

Installation

npm install pm2 -g

Start an application

pm2 start app.js

f) serve-favicon

Node.js middleware for serving a favicon. The serve-favicon module lets us exclude requests for the favicon in our logger middleware. It also caches the icon in memory to improve performance by reducing disk access. In addition, it provides an ETag based on the contents of the icon, rather than file system properties.

Installation

npm install serve-favicon

Example:

var express = require('express')
var favicon = require('serve-favicon')
var path = require('path')

var app = express()
app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')))

// Add your routes here, etc.

app.listen(3000)

g) cors

Cross-Origin Resource Sharing (CORS) headers allow apps running in the browser to make requests to servers on different domains (also known as origins). CORS headers are set on the server side – the HTTP server is responsible for indicating that a given HTTP request can be cross-origin. CORS defines a way in which a browser and server can interact and determine whether or not it is safe to allow a cross-origin request.

Installation

npm install cors

Example: Enable All CORS Requests

var express = require('express')
var cors = require('cors')
var app = express()

app.use(cors())

app.get('/products/:id', function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for all origins!'})
})

app.listen(8080, function () {
  console.log('CORS-enabled web server listening on port 80')
})

Example: Enable CORS for a Single Route

var express = require('express')
var cors = require('cors')
var app = express()

app.get('/products/:id', cors(), function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for a Single Route'})
})

app.listen(8080, function () {
  console.log('CORS-enabled web server listening on port 80')
})

h) dotenv

When a NodeJs application runs, it injects a global variable called process.env which contains information about the state of environment in which the application is running. The dotenv loads environment variables stored in the .env file into process.env.

Installation

npm install dotenv

Usage

// .env

DB_HOST=localhost
DB_USER=admin
DB_PASS=root
// config.js

const db = require('db')

db.connect({
  host: process.env.DB_HOST,
  username: process.env.DB_USER,
  password: process.env.DB_PASS
})

i) fs-extra

fs-extra contains methods that aren’t included in the vanilla Node.js fs package. Such as recursive mkdircopy, and remove. It also uses graceful-fs to prevent EMFILE errors.

Installation

npm install fs-extra

Usage

const fs = require('fs-extra')


// Async with callbacks:
fs.copy('/tmp/myfile', '/tmp/mynewfile', err => {
  if (err) return console.error(err)
  console.log('success!')
})

j) moment

A JavaScript date library for parsing, validating, manipulating, and formatting dates.

Installation

npm install moment --save

Usage

  • Format Dates
const moment = require('moment');

moment().format('MMMM Do YYYY, h:mm:ss a'); // October 24th 2020, 3:15:22 pm
moment().format('dddd');                    // Saturday
moment().format("MMM Do YY");               // Oct 24th 20

Relative Time

const moment = require('moment');

moment("20111031", "YYYYMMDD").fromNow(); // 9 years ago
moment("20120620", "YYYYMMDD").fromNow(); // 8 years ago
moment().startOf('day').fromNow();        // 15 hours ago

Calendar Time

const moment = require('moment');

moment().subtract(10, 'days').calendar(); // 10/14/2020
moment().subtract(6, 'days').calendar();  // Last Sunday at 3:18 PM
moment().subtract(3, 'days').calendar();  // Last Wednesday at 3:18 PM

How does routing work in Node.js?

Routing defines the way in which the client requests are handled by the application endpoints. We define routing using methods of the Express app object that correspond to HTTP methods; for example, app.get() to handle GET requests and app.post to handle POST requests, app.all() to handle all HTTP methods and app.use() to specify middleware as the callback function.

These routing methods “listens” for requests that match the specified route(s) and method(s), and when it detects a match, it calls the specified callback function.

Syntax:

app.METHOD(PATH, HANDLER)

Where:

  • app is an instance of express.
  • METHOD is an HTTP request method.
  • PATH is a path on the server.
  • HANDLER is the function executed when the route is matched.

a) Route methods

// GET method route
app.get('/', function (req, res) {
  res.send('GET request')
})

// POST method route
app.post('/login', function (req, res) {
  res.send('POST request')
})

// ALL method route
app.all('/secret', function (req, res, next) {
  console.log('Accessing the secret section ...')
  next() // pass control to the next handler
})

b) Route paths

Route paths, in combination with a request method, define the endpoints at which requests can be made. Route paths can be strings, string patterns, or regular expressions.

The characters ?+*, and () are subsets of their regular expression counterparts. The hyphen (-) and the dot (.) are interpreted literally by string-based paths.

Example:

// This route path will match requests to /about.
app.get('/about', function (req, res) {
  res.send('about')
})


// This route path will match acd and abcd.
app.get('/ab?cd', function (req, res) {
  res.send('ab?cd')
})


// This route path will match butterfly and dragonfly
app.get(/.*fly$/, function (req, res) {
  res.send('/.*fly$/')
})

c) Route parameters

Route parameters are named URL segments that are used to capture the values specified at their position in the URL. The captured values are populated in the req.params object, with the name of the route parameter specified in the path as their respective keys.

Example:

app.get('/users/:userId', function (req, res) {
  res.send(req.params)
})

Response methods

MethodDescription
res.download()Prompt a file to be downloaded.
res.end()End the response process.
res.json()Send a JSON response.
res.jsonp()Send a JSON response with JSONP support.
res.redirect()Redirect a request.
res.render()Render a view template.
res.send()Send a response of various types.
res.sendFile()Send a file as an octet stream.
res.sendStatus()Set the response status code and send its string representation as the response body.

d) Router method

var express = require('express')
var router = express.Router()

// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
  console.log('Time: ', Date.now())
  next()
})

// define the home page route
router.get('/', function (req, res) {
  res.send('Birds home page')
})

// define the about route
router.get('/about', function (req, res) {
  res.send('About birds')
})

module.exports = router

How node.js prevents blocking code?

Blocking vs Non-blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

Synchronous methods in the Node.js standard library that use Libuv are the most commonly used blocking operations. Native modules may also have blocking methods. Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Example:

// Blocking
const fs = require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read
console.log(data);
moreWork(); // will run after console.log

// Non-blocking
const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
  console.log(data);
});
moreWork(); // will run before console.log

Node JS Interview Question

What is difference between promises and async-await in Node.js?

1. Promises

A promise is used to handle the asynchronous result of an operation. JavaScript is designed to not wait for an asynchronous block of code to completely execute before other synchronous parts of the code can run. With Promises, we can defer the execution of a code block until an async request is completed. This way, other operations can keep running without interruption.

States of Promises:

  • Pending: Initial State, before the Promise succeeds or fails.
  • Resolved: Completed Promise
  • Rejected: Failed Promise, throw an error

Example:

function logFetch(url) {
  return fetch(url)
    .then(response => {
      console.log(response);
    })
    .catch(err => {
      console.error('fetch failed', err);
    });
}

2. Async-Await

Await is basically syntactic sugar for Promises. It makes asynchronous code look more like synchronous/procedural code, which is easier for humans to understand.

Putting the keyword async before a function tells the function to return a Promise. If the code returns something that is not a Promise, then JavaScript automatically wraps it into a resolved promise with that value. The await keyword simply makes JavaScript wait until that Promise settles and then returns its result.

Example:

async function logFetch(url) {
  try {
    const response = await fetch(url);
    console.log(response);
  }
  catch (err) {
    console.log('fetch failed', err);
  }
}

How to use JSON Web Token (JWT) for authentication in Node.js?

JSON Web Token (JWT) is an open standard that defines a compact and self-contained way of securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed.

There are some advantages of using JWT for authorization:

  • Purely stateless. No additional server or infra required to store session information.
  • It can be easily shared among services.

JSON Web Tokens consist of three parts separated by dots (.), which are:

jwt.sign(payload, secretOrPrivateKey, [options, callback])
  • Header – Consists of two parts: the type of token (i.e., JWT) and the signing algorithm (i.e., HS512)
  • Payload – Contains the claims that provide information about a user who has been authenticated along with other information such as token expiration time.
  • Signature – Final part of a token that wraps in the encoded header and payload, along with the algorithm and a secret

Installation

npm install jsonwebtoken bcryptjs --save

Example: AuthController.js

var express = require('express');
var router = express.Router();
var bodyParser = require('body-parser');
var User = require('../user/User');

var jwt = require('jsonwebtoken');
var bcrypt = require('bcryptjs');
var config = require('../config');


router.use(bodyParser.urlencoded({ extended: false }));
router.use(bodyParser.json());

router.post('/register', function(req, res) {
  
  var hashedPassword = bcrypt.hashSync(req.body.password, 8);
  
  User.create({
    name : req.body.name,
    email : req.body.email,
    password : hashedPassword
  },
  function (err, user) {
    if (err) return res.status(500).send("There was a problem registering the user.")
    // create a token
    var token = jwt.sign({ id: user._id }, config.secret, {
      expiresIn: 86400 // expires in 24 hours
    });
    res.status(200).send({ auth: true, token: token });
  });
});

config.js

// config.js
module.exports = {
  'secret': 'supersecret'
};

The jwt.sign() method takes a payload and the secret key defined in config.js as parameters. It creates a unique string of characters representing the payload. In our case, the payload is an object containing only the id of the user.

How to build a microservices architecture with Node.js?

Microservices

Microservices are a style of service-oriented architecture (SOA) where the app is structured on an assembly of interconnected services. With microservices, the application architecture is built with lightweight protocols. The services are finely seeded in the architecture. Microservices disintegrate the app into smaller services and enable improved modularity.

Microservices

There are few things worth emphasizing about the superiority of microservices, and distributed systems generally, over monolithic architecture:

  • Modularity — responsibility for specific operations is assigned to separate pieces of the application
  • Uniformity — microservices interfaces (API endpoints) consist of a base URI identifying a data object and standard HTTP methods (GET, POST, PUT, PATCH and DELETE) used to manipulate the object
  • Robustness — component failures cause only the absence or reduction of a specific unit of functionality
  • Maintainability — system components can be modified and deployed independently
  • Scalability — instances of a service can be added or removed to respond to changes in demand.
  • Availability — new features can be added to the system while maintaining 100% availability.
  • Testability — new solutions can be tested directly in the production environment by implementing them for restricted segments of users to see how they behave in real life.

Creating Microservices with Node.js

Step 01: Creating a Server to Accept Requests

This file is creating our server and assigns routes to process all requests.

//  server.js

const express = require('express')
const app = express();
const port = process.env.PORT || 3000;

const routes = require('./api/routes');
routes(app);
app.listen(port, function() {
   console.log('Server started on port: ' + port);
});

Step 02: Defining the routes

The next step is to define the routes for the microservices and then assign each to a target in the controller. We have two endpoints. One endpoint called “about” that returns information about the application. And a “distance” endpoint that includes two path parameters, both Zip Codes of the Lego store. This endpoint returns the distance, in miles, between these two Zip Codes.

const controller = require('./controller');

module.exports = function(app) {
   app.route('/about')
       .get(controller.about);
   app.route('/distance/:zipcode1/:zipcode2')
       .get(controller.getDistance);
};

Step 03: Adding Controller Logic

Within the controller file, we are going to create a controller object with two properties. Those properties are the functions to handle the requests we defined in the routes module.

var properties = require('../package.json')
var distance = require('../service/distance');

var controllers = {
   about: function(req, res) {
       var aboutInfo = {
           name: properties.name,
           version: properties.version
       }
       res.json(aboutInfo);
   },
   getDistance: function(req, res) {
           distance.find(req, res, function(err, dist) {
               if (err)
                   res.send(err);
               res.json(dist);
           });
       },
};

module.exports = controllers;

Advance Node JS Interview Question

How to use Q promise in Node.js?

A promise is an object that represents the return value or the thrown exception that the function may eventually provide. A promise can also be used as a proxy for a remote object to overcome latency.

Promise is relatively an easy implementation for asynchronous operation. The promise object returned from the function represents an operation which is not completed yet, but it guarantees to the caller of the operation that the operation will be completed in future.

Promise has the following states:

  • Pending – asynchronous operation is not yet completed.
  • Fulfilled – asynchronous operation is completed successfully.
  • Rejected – asynchronous operation is terminated with an error.
  • Settled – asynchronous operation is either fulfilled or rejected.
  • Callback – function is executed if the promise is executed with value.
  • Errback – function is executed if the promise is rejected.

Moving to Promises from Callback

On the first pass, promises can mitigate the Pyramid of Doom: the situation where code marches to the right faster than it marches forward.

step1(function (value1) {
    step2(value1, function(value2) {
        step3(value2, function(value3) {
            step4(value3, function(value4) {
                // Do something with value4
            });
        });
    });
});

With a promise library, it can flatten the pyramid.

Q.fcall(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
    // Do something with value4
})
.catch(function (error) {
    // Handle any error from all above steps
})
.done();

How to implement Memcached in Node.js?

Memcached is a general-purpose distributed memory caching system. It is often used to speed up dynamic database-driven websites by caching data and objects in RAM to reduce the number of times an external data source (such as a database or API) must be read. Memcached is free and open-source software, licensed under the Revised BSD licence. Memcached runs on Unix-like operating systems (at least LINUX and OS X) and on Microsoft windows.

We can store data to memcached server in key pair format. So whenever any request come from the app can be matched with memcached server without any query from mysql/Nosql server. This increases the performance of the application.

Installation

npm install memcached

Setting up the client

The constructor of the memcached client take 2 different arguments server locations and options. Syntax:

var Memcached = require('memcached');
var memcached = new Memcached(Server locations, options);

Example usage:

var Memcached = require('memcached');
// all global configurations should be applied to the .config object of the Client.
Memcached.config.poolSize = 25;

var memcached = new Memcached('localhost:11211', {retries:10,retry:10000,remove:true,failOverServers:['192.168.0.103:11211']});

How to generate and verify checksum of the given string in Nodejs

The checksum (aka hash sum) calculation is a one-way process of mapping an extensive data set of variable length (e.g., message, file), to a smaller data set of a fixed length (hash). The length depends on a hashing algorithm.

For the checksum generation, we can use node crypto() module. The module uses createHash(algorithm) to create a checksum (hash) generator. The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform.

Example:

const crypto = require('crypto');

// To get a list of all available hash algorithms
crypto.getHashes() // [ 'md5', 'sha1', 'sha3-256', ... ]

  
// Create hash of SHA1 type
const key = "MY_SECRET_KEY";


// 'digest' is the output of hash function containing  
// only hexadecimal digits
hashPwd = crypto.createHash('sha1').update(key).digest('hex');
  
console.log(hashPwd); //ef5225a03e4f9cc953ab3c4dd41f5c4db7dc2e5b

Node JS Interview Question

What is Chaining in Node?

Chanining is a mechanism to connect output of one stream to another stream and create a chain of multiple stream operations. It is normally used with piping operations.

Why Node.js is single threaded?

For async processing, Node.js was created explicitly as an experiment. It is believed that more performance and scalability can be achieved by doing async processing on a single thread under typical web loads than the typical thread based implementation.

Node JS Part 1Node JS Part 2
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