Notes

********node js
Node.js is an open-source, cross-platform JavaScript runtime environment and library to run web applications outside the client’s browser. It is used to create server-side web applications.

Node.js is perfect for data-intensive applications as it uses an asynchronous, event-driven model. You can use I/O intensive web applications like video streaming sites. You can also use it for developing: Real-time web applications, Network applications, General-purpose applications, and Distributed systems.
************node js work flow :
Clients send requests to the webserver to interact with the web application. Requests can be non-blocking or blocking:
Querying for data
Deleting data
Updating the data
Node.js retrieves the incoming requests and adds those to the Event Queue
The requests are then passed one-by-one through the Event Loop. It checks if the requests are simple enough not to require any external resources
The Event Loop processes simple requests (non-blocking operations), such as I/O Polling, and returns the responses to the corresponding clients
A single thread from the Thread Pool is assigned to a single complex request. This thread is responsible for completing a particular blocking request by accessing external resources, such as computation, database, file system, etc.

Once the task is carried out completely, the response is sent to the Event Loop that sends that response back to the client.
*************Callbacks:
A callback function is called after a given task. It allows other code to be run in the meantime and prevents any blocking. Being an asynchronous platform, Node.js heavily relies on callback. All APIs of Node are written to support callback
***************NPM:
NPM stands for Node Package Manager, responsible for managing all the packages and modules for Node.js.

Node Package Manager provides two main functionalities:

Provides online repositories for node.js packages/modules, which are searchable on search.nodejs.org
Provides command-line utility to install Node.js packages and also manages Node.js versions and dependencies
**************Modules :
Modules are like JavaScript libraries that can be used in a Node.js application to include a set of functions. To include a module in a Node.js application, use the require() function with the parentheses containing the module's name.
***************What is the purpose of the module .Exports?
In Node.js, a module encapsulates all related codes into a single unit of code that can be parsed by moving all relevant functions into a single file. You may export a module with the module and export the function, which lets it be imported into another file with a needed keyword.
************** Node.js preferred over other backend technologies like Java and PHP?
Some of the reasons why Node.js is preferred include:

-Node.js is very fast
-Node Package Manager has over 50,000 bundles available at the developer’s disposal
-Perfect for data-intensive, real-time web applications, as Node.js never waits for an API to return data
-Better synchronization of code between server and client due to same code base
-Easy for web developers to start using Node.js in their projects as it is a JavaScript library
**************MongoDB
MongoDB is the most common database used with Node.js. It is a NoSQL, cross-platform, document-oriented database that provides high performance, high availability, and easy scalability.
*************** What are the pros and cons of Node.js?
Node.js Pros

Node.js Cons

P-Fast processing and an event-based model

C-Not suitable for heavy computational tasks

P-Uses JavaScript, which is well-known amongst developers

C-Using callback is complex since you end up with several nested callbacks

Node Package Manager has over 50,000 packages that provide the functionality to an application

Dealing with relational databases is not a good option for Node.js

Best suited for streaming huge amounts of data and I/O intensive operations

Since Node.js is single-threaded, CPU intensive tasks are not its strong suit
****************Event Loop in Node.js?
Event loops handle asynchronous callbacks in Node.js. It is the foundation of the non-blocking input/output in Node.js, making it one of the most important environmental features.
*************URL module in Node.js?
The URL module in Node.js splits up a web address into readable parts. Use require() to include the module:
var url = require('url');
//The URL module in Node.js provides various utilities for URL resolution and parsing. It is a built-in module that helps split up the web address into a readable format.
************asynchronous and non-blocking APIs in Node.js.
All Node.js library APIs are asynchronous, which means they are also non-blocking
A Node.js-based server never waits for an API to return data. Instead, it moves to the next API after calling it, and a notification mechanism from a Node.js event responds to the server for the previous API call
***********callback hell?
Callback hell, also known as the pyramid of doom, is the result of intensively nested, unreadable, and unmanageable callbacks, which in turn makes the code harder to read and debug
improper implementation of the asynchronous logic causes callback hell
************middleware in Node.js.
Middleware is a function that receives the request and response objects. Most tasks that the middleware functions perform are:

Execute any code
Update or modify the request and the response objects
Finish the request-response cycle
Invoke the next middleware in the stack
******************Node.js overcome the problem of blocking of I/O operations?
Since the node has an event loop that can be used to handle all the I/O operations in an asynchronous manner without blocking the main function.

So for example, if some network call needs to happen it will be scheduled in the event loop instead of the main thread(single thread). And if there are multiple such I/O calls each one will be queued accordingly to be executed separately(other than the main thread).

Thus even though we have single-threaded JS, I/O ops are handled in a nonblocking way.

****************NODE_ENV?
NODE_ENV is an environment variable made popular by the express web server framework. When a node application is run, it can check the value of the environment variable and do different things based on the value.

For example, when we work on a project and there are production and development environments. We don't need to use caching in the development env. So we set

$ NODE_ENV=development
and use the code below

if (process.env.NODE_ENV === 'development')
useCaching = false;
Upon that, if the project runs on production it will use caching.
**********************Promises in Node.js?
It allows to associate handlers to an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of the final value, the asynchronous method returns a promise for the value at some point in the future.

Promises in node.js promised to do some work and then had separate callbacks that would be executed for success and failure as well as handling timeouts. Another way to think of promises in node.js was that they were emitters that could emit only two events: success and error.The cool thing about promises is you can combine them into dependency chains (do Promise C only when Promise A and Promise B complete).

The core idea behind promises is that a promise represents the result of an asynchronous operation. A promise is in one of three different states:

pending - The initial state of a promise.
fulfilled - The state of a promise representing a successful operation.
rejected - The state of a promise representing a failed operation. Once a promise is fulfilled or rejected, it is immutable (i.e. it can never change again).
Creating a Promise

var myPromise = new Promise(function(resolve, reject){
....
})
*******************Node.js is a single threaded
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.

Nodejs Primary application runs in an event loop, which is in a single thread.
Background I/O is running in a thread pool that is only accessible to C/C++ or other compiled/native modules and mostly transparent to the JS.
Node v11/12 now has experimental worker_threads, which is another option.
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.

***********************Express.js?
ExpressJS 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;

MongoDB - The standard NoSQL database
Express.js - The default web applications framework
Angular.js - The JavaScript MVC framework used for web applications
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() {})
*************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.
******************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.
**************************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

********************Indexing in MongoDB
MongoDB uses indexing in order to make the query processing more efficient. If there is no indexing, then the MongoDB must scan every document in the collection and retrieve only those documents that match the query. Indexes are special data structures that stores some information related to the documents such that it becomes easy for MongoDB to find the right data file. The indexes are order by the value of the field specified in the index.
Creating an Index :
MongoDB provides a method called createIndex() that allows user to create an index.