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What Are Callbacks in Node.JS

The callback is an asynchronous equivalent for a function. It is called following every task. In Node.js, callbacks are frequently used. All APIs of Node are drafted in a way that supports callbacks.  To illustrate, when a function commences reading a file, it instantly returns the control to the execution environment to execute the succeeding instruction.In Node.js, once file I/O is concluded, it would call the callback function. There is no hindrance or waiting for File I/O. This presents Node.js as highly scalable, as it can process an extraordinary number of requests without pausing for any function to return results.Understanding the callback pattern for asynchronous programmingIn the asynchronous programming model, something happens one at a time. An asynchronous model acknowledges multiple things to occur at the same time. When you commence an action, your program proceeds to run. When the step completes, the program is acquainted and gets access to the result (for example, the data read from disk).Node.js encourages an asynchronous coding method from the ground up, in contrast to several popular web frameworks. There are numerous vital things to be conscious of when learning to write asynchronous code – and at the same time, you will frequently find your code executing in highly unexpected ways.Various functions in Node.js core have both synchronous and asynchronous versions. It will be far better to use asynchronous operations in most conditions; otherwise, why use Node.js?Asynchronous continuation-passing techniquelet's consider an example where a function is asynchronous, which is as follows:function exampleAsync(a, b, callback) {   setTimeout(function() {     callback(a + b);   }, 100); } console.log('Before asynchronous call’); exampleAsync(2, 3, function(finalresult)   {     console.log('Result: ' + finalresult); }); console.log('After asynchronous call');OutPut:Before asynchronous call After asynchronous call Result: 5setTimeout() triggers an asynchronous process; it will not wait for the callback to get executed. It returns quickly, providing the control back to exampleAsync(), and then back to its caller.The following image shows how this works:Examples of the callback pattern in Node.jsA callback is a function called when the task finishes, and a callback function allows other code to run in the meantime. Using the Callback concept, Node.js can process many requests without waiting for any function to return the result, making Node.js highly scalable. For example: In Node.js, when a function starts reading the file, it returns the control to the execution environment immediately to execute the next instruction. Once file I/O gets completed, the callback function will get called to avoid blocking or wait for File I/O.Example 1: Reading a file synchronously in Node.js. Create a text file synch.txt with the following content:Hello, this is my first testing of synchronous content.Create a first.js file:var fs = require("fs"); var datatxt = fs.readFileSync('synch.txt'); console.log(datatxt.toString()); console.log("Execution ends");Output:Hello, this is my first testing of synchronous content.Execution endsInformation: The fs library is loaded to handle file-system associated operations. The readFileSync() function is synchronous and blocks execution till terminated. The function blocks the program until it reads the file, and then only it proceeds to end the program.Example 2: Reading a file asynchronously in Node.js. Create a text file asynch.txt with the content asHello, this is my first testing of asynchronous content.var fs = require("fs");     fs.readFile('asynch.txt', function (ferr, dataasynch) {       if (ferr) return console.error(ferr);       console.log(dataasynch.toString());   });   console.log("Execution ends"); Output:Execution endsHello, this is my first testing of asynchronous content.Information: The fs library is loaded to handle file-system-related operations. The readFile() function is asynchronous, and the control returns immediately to the next instruction in the program while the function keeps running in the background. A callback function is relinquished, which gets called when the task running in the background is finished.Getting trapped in callback hellCallback Hell is an anti-pattern detected in the code of asynchronous programming. It is a slang phrase applied to define a cumbersome number of nested “if” statements or functions. If the application logic gets complex, a few callbacks appear harmless. But once your project demands grow, it is common to fall under piling layers of nested callbacks.The callback is a function where “A” is passed to another function, “B,” as a parameter. The function “B” then executes the code “A” at some point. The invocation of “A” can be immediate, as in a synchronous callback, or it can occur later as in an asynchronous callback.var callbackhell = require(‘fs’) callbackhell.readFile(‘test.json’, function(‘err’, results){ if(err){ console.log(err); } console.log(JSON.parse(results).name) });In the code, we call readFile and pass it as a second parameter function (Callback Hell). readFile will execute the callback bypassing the procedure's results to parameters.The use of callbacks makes the code tedious to write and manage. It increases the difficulty of identifying the application's flow, which is an obstacle, hence the popular name of Callback Hell.What’s more dangerous than callback hell?Not fixing the nested callback hellUsing Promises to write asynchronous codePromises are the function that a value would be returned at a later time. Instead of returning concrete values, these asynchronous functions return a Promise object, which will at some point either be fulfilled or not.A promise represents an asynchronous operation. It means a process that has not been completed yet but is expected to in the future. Let's have a look at a simple file read example without using promises:fs.readFile(filePath, (err, result) => {      if (err) { console.log(err); }      console.log(data); }); if the readFile function returned a promise, the logic would be  written as below var fileReadandPromise = fs.readFile(filePath); fileReadandPromise.then(console.log, console.error)The fileReadandPromise is passed multiple times in a code where you need to read a file. This helps in writing robust unit tests for your code since you now only have to write a single test for a promise. And more readable code!Promise.all()The Promise. all() method accepts an iterable of promises as an input and returns a single Promise that fixes to an array of the input promises' results.const promise1 = Promise.resolve(5); const promise2 = 54; const promise3 = new Promise((resolve, reject) => {   setTimeout(resolve, 100, 'foo'); }); Promise.all([promise1, promise2, promise3]).then((values) => {   console.log(values); });Promise.any()Promise.any() takes an iterable of Promise objects and, as soon as one of the promises in the iterable fulfills, returns a single promise that resolves with the value from that promise. If there are no promises in the iterable fulfill, then the returned promise is rejected with an AggregateError, a new subclass of Error that groups together individual errors. This method is the opposite of Promise.all().const promiseand1 = Promise.reject(0); const promiseand2 = new Promise((resolve) => setTimeout(resolve, 100, 'Large')); const promiseand3 = new Promise((resolve) => setTimeout(resolve, 500, 'Small')); const promises = [promiseand1, promiseand2, promiseand3]; Promise.any(promises).then((value) => console.log(value));Using Async / Await for handling asynchronous codeInitial versions of Node didn't have the Node architecture single-threaded and asynchronous. The intricacy with this kind of code is that this kind of position can create many problems, and the code can get messy when there are several functions, and this situation is called callback hell.Promises and function chaining were introduced to overcome this situation.By Node v8, the async/await feature was finally wheeled out to deal with Promises and function chaining. The functions were not necessarily required to be chained after another; they simply await the function that returns the Promise. But the async function is needed to be declared before awaiting a function returning a Promise.Examples of Async / AwaitThe code looks like the following.async function testfun1(req, res){   let resp1 = await request.get('http://localhost:8080');     if (resp1.err) { console.log('error');}     else { console.log('Response Fetched'); }ExplanationThe code above essentially demands the JavaScript engine driving the code to wait for the request.get() function to accomplish before moving on to the next line to execute it. The request.get() function returns a Promise for which the user will await. Before async/await, if it is required to check that the functions are running in the desired sequence, i.e. one after the another, chain them one after the another or register callbacks. Code review and understanding become comfortable with async/await, as observed from the above example.Error handling in the case of async / awaitFormulating exceptionsAn exception is built using the throw keyword:throw valueAs soon as the above line executes, the standard program flow stops and the control is held back to the most imminent exception handler.Typically, in client-side code, a value can be any JavaScript value, including a string, a number or an object.In Node.js, we don't throw strings; we throw Error objects.Error objectsAn error object is an object that is either an instance of the Error object or extends the Error class provided in the Error core module:throw new Error('Out of Mall') OR class NoPersoninMallError extends Error {   //... } throw new NoPersoninMallError()Exception handlingAn exception handler is a try/catch statement.Any exception created in the lines of code in the try block is as below:try { //code would be written here. } catch (e) {} //e is the exception value. Error handling with async/awaitUsing async/await, the errors can be caught as below:async function AnyFunction() {   try {     await anotherFunction()   } catch (err) {     console.error(err.message)   } } ConclusionNode.js is more beneficial to the developers in association with its disadvantages. What’s more important is that it has extended the JavaScript applications area and can be used for both front-end and back-end servers.Node.js is, without a doubt, one of the more exciting technologies in use today, and it has grown into one of the most popular platforms used for web applications, services, and desktop apps. With time, more and more business organizations have opted to use Node.js and are consistently getting positive results.

What Are Callbacks in Node.JS

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What Are Callbacks in Node.JS

The callback is an asynchronous equivalent for a function. It is called following every task. In Node.js, callbacks are frequently used. All APIs of Node are drafted in a way that supports callbacks.  To illustrate, when a function commences reading a file, it instantly returns the control to the execution environment to execute the succeeding instruction.

In Node.js, once file I/O is concluded, it would call the callback function. There is no hindrance or waiting for File I/O. This presents Node.js as highly scalable, as it can process an extraordinary number of requests without pausing for any function to return results.

Understanding the callback pattern for asynchronous programming

In the asynchronous programming model, something happens one at a time. An asynchronous model acknowledges multiple things to occur at the same time. When you commence an action, your program proceeds to run. When the step completes, the program is acquainted and gets access to the result (for example, the data read from disk).

Node.js encourages an asynchronous coding method from the ground up, in contrast to several popular web frameworks. There are numerous vital things to be conscious of when learning to write asynchronous code – and at the same time, you will frequently find your code executing in highly unexpected ways.

Various functions in Node.js core have both synchronous and asynchronous versions. It will be far better to use asynchronous operations in most conditions; otherwise, why use Node.js?

Asynchronous continuation-passing technique

let's consider an example where a function is asynchronous, which is as follows:

function exampleAsync(a, b, callback) {
  setTimeout(function() {
    callback(a + b);
  }, 100);
}
console.log('Before asynchronous call’);
exampleAsync(2, 3, function(finalresult)  
{
    console.log('Result: ' + finalresult);
}); console.log('After asynchronous call');

OutPut:

Before asynchronous call 
After asynchronous call 
Result: 5

setTimeout() triggers an asynchronous process; it will not wait for the callback to get executed. It returns quickly, providing the control back to exampleAsync(), and then back to its caller.

The following image shows how this works:

Asynchronous continuation-passing technique

Examples of the callback pattern in Node.js

A callback is a function called when the task finishes, and a callback function allows other code to run in the meantime. Using the Callback concept, Node.js can process many requests without waiting for any function to return the result, making Node.js highly scalable. For example: In Node.js, when a function starts reading the file, it returns the control to the execution environment immediately to execute the next instruction. Once file I/O gets completed, the callback function will get called to avoid blocking or wait for File I/O.

Example 1: Reading a file synchronously in Node.js. Create a text file synch.txt with the following content:

Hello, this is my first testing of synchronous content.

Create a first.js file:

var fs = require("fs"); 
var datatxt = fs.readFileSync('synch.txt'); 
console.log(datatxt.toString()); 
console.log("Execution ends");

Output:

Hello, this is my first testing of synchronous content.
Execution ends

Information: The fs library is loaded to handle file-system associated operations. The readFileSync() function is synchronous and blocks execution till terminated. The function blocks the program until it reads the file, and then only it proceeds to end the program.

Example 2: Reading a file asynchronously in Node.js. Create a text file asynch.txt with the content as

Hello, this is my first testing of asynchronous content.

var fs = require("fs");     
fs.readFile('asynch.txt', function (ferr, dataasynch) {  
    if (ferr) return console.error(ferr);  
    console.log(dataasynch.toString());  
});  
console.log("Execution ends"); 

Output:

Execution ends
Hello, this is my first testing of asynchronous content.

Information: The fs library is loaded to handle file-system-related operations. The readFile() function is asynchronous, and the control returns immediately to the next instruction in the program while the function keeps running in the background. A callback function is relinquished, which gets called when the task running in the background is finished.

Getting trapped in callback hell

Callback Hell is an anti-pattern detected in the code of asynchronous programming. It is a slang phrase applied to define a cumbersome number of nested “if” statements or functions. If the application logic gets complex, a few callbacks appear harmless. But once your project demands grow, it is common to fall under piling layers of nested callbacks.

The callback is a function where “A” is passed to another function, “B,” as a parameter. The function “B” then executes the code “A” at some point. The invocation of “A” can be immediate, as in a synchronous callback, or it can occur later as in an asynchronous callback.

var callbackhell = require(‘fs’)
callbackhell.readFile(‘test.json’, function(‘err’, results){
if(err){
console.log(err);
}
console.log(JSON.parse(results).name)
});

In the code, we call readFile and pass it as a second parameter function (Callback Hell). readFile will execute the callback bypassing the procedure's results to parameters.

The use of callbacks makes the code tedious to write and manage. It increases the difficulty of identifying the application's flow, which is an obstacle, hence the popular name of Callback Hell.

What’s more dangerous than callback hell?

Not fixing the nested callback hell

Using Promises to write asynchronous code

Promises are the function that a value would be returned at a later time. Instead of returning concrete values, these asynchronous functions return a Promise object, which will at some point either be fulfilled or not.

A promise represents an asynchronous operation. It means a process that has not been completed yet but is expected to in the future. Let's have a look at a simple file read example without using promises:

fs.readFile(filePath, (err, result) => {
     if (err) { console.log(err); }
     console.log(data);
});
if the readFile function returned a promise, the logic would be  written as below
var fileReadandPromise = fs.readFile(filePath);
fileReadandPromise.then(console.log, console.error)

The fileReadandPromise is passed multiple times in a code where you need to read a file. This helps in writing robust unit tests for your code since you now only have to write a single test for a promise. And more readable code!

Promise.all()

The Promise. all() method accepts an iterable of promises as an input and returns a single Promise that fixes to an array of the input promises' results.

const promise1 = Promise.resolve(5);
const promise2 = 54;
const promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
});
Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values);
});

Promise.any()

Promise.any() takes an iterable of Promise objects and, as soon as one of the promises in the iterable fulfills, returns a single promise that resolves with the value from that promise. If there are no promises in the iterable fulfill, then the returned promise is rejected with an AggregateError, a new subclass of Error that groups together individual errors. This method is the opposite of Promise.all().

const promiseand1 = Promise.reject(0);
const promiseand2 = new Promise((resolve) => setTimeout(resolve, 100, 'Large'));
const promiseand3 = new Promise((resolve) => setTimeout(resolve, 500, 'Small'));
const promises = [promiseand1, promiseand2, promiseand3];
Promise.any(promises).then((value) => console.log(value));

Using Async / Await for handling asynchronous code

Initial versions of Node didn't have the Node architecture single-threaded and asynchronous. The intricacy with this kind of code is that this kind of position can create many problems, and the code can get messy when there are several functions, and this situation is called callback hell.

Promises and function chaining were introduced to overcome this situation.

By Node v8, the async/await feature was finally wheeled out to deal with Promises and function chaining. The functions were not necessarily required to be chained after another; they simply await the function that returns the Promise. But the async function is needed to be declared before awaiting a function returning a Promise.

Examples of Async / Await

The code looks like the following.

async function testfun1(req, res){
  let resp1 = await request.get('http://localhost:8080');
    if (resp1.err) { console.log('error');}
    else { console.log('Response Fetched');
}

Explanation

The code above essentially demands the JavaScript engine driving the code to wait for the request.get() function to accomplish before moving on to the next line to execute it. The request.get() function returns a Promise for which the user will await. Before async/await, if it is required to check that the functions are running in the desired sequence, i.e. one after the another, chain them one after the another or register callbacks. 

Code review and understanding become comfortable with async/await, as observed from the above example.

Error handling in the case of async / await

Formulating exceptions
An exception is built using the throw keyword:
throw value
As soon as the above line executes, the standard program flow stops and the control is held back to the most imminent exception handler.

Typically, in client-side code, a value can be any JavaScript value, including a string, a number or an object.

In Node.js, we don't throw strings; we throw Error objects.

Error objects
An error object is an object that is either an instance of the Error object or extends the Error class provided in the Error core module:

throw new Error('Out of Mall')
OR
class NoPersoninMallError extends Error {
  //...
}
throw new NoPersoninMallError()

Exception handling

An exception handler is a try/catch statement.
Any exception created in the lines of code in the try block is as below:

try { 
//code would be written here. 
} catch (e) {} 
//e is the exception value. 

Error handling with async/await

Using async/await, the errors can be caught as below:

async function AnyFunction() { 
  try { 
    await anotherFunction() 
  } catch (err) { 
    console.error(err.message) 
  } 
}

Conclusion

Node.js is more beneficial to the developers in association with its disadvantages. What’s more important is that it has extended the JavaScript applications area and can be used for both front-end and back-end servers.

Node.js is, without a doubt, one of the more exciting technologies in use today, and it has grown into one of the most popular platforms used for web applications, services, and desktop apps. With time, more and more business organizations have opted to use Node.js and are consistently getting positive results.

Rajesh

Rajesh Bhagia

Blog Author

Rajesh Bhagia is experienced campaigner in Lamp technologies and has 10 years of experience in Project Management. He has worked in Multinational companies and has handled small to very complex projects single-handedly. He started his career as Junior Programmer and has evolved in different positions including Project Manager of Projects in E-commerce Portals. Currently, he is handling one of the largest project in E-commerce Domain in MNC company which deals in nearly 9.5 million SKU's.

In his role as Project Manager at MNC company, Rajesh fosters an environment of teamwork and ensures that strategy is clearly defined while overseeing performance and maintaining morale. His strong communication and client service skills enhance his process-driven management philosophy.

Rajesh is a certified Zend Professional and has developed a flair for implementing PMP Knowledge Areas in daily work schedules. He has well understood the importance of these process and considers that using the knowledge Areas efficiently and correctly can turn projects to success. He also writes articles/blogs on Technology and Management

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The event handling in react is declarative and the advantage of declarative way to handlers is that they are part of the User interface structure.  Let’s take a look at event handling in class components                       Import React, { Component } from ‘...react’                         class TestApp extends Component {                              clickHandler() {                                  console.log(“clicked”)                                }                                render(){                                      return(                                                                                     Click me                                                                                 )                                 }                            }                       export default TestApp You cannot return false to prevent default behaviour in React. You must call preventDefault explicitly.  In HTML it looks like below:    Click Output: It will print “Clicked”  And in React, like this:  function clickHandle(e) {       e.preventDefault();       console.log(“Handled”);   }  Click  Output : console will print “Handled”  There are some  event handlers triggered by an event in the bubbling phase which is the same as with the normal DOM API; simply attach a handler to an eventual parent of an element and any events triggered on that element will bubble to the parent as long as it's not stopped via stopPropagation along the way   Click me  Below are some of the event handlers triggered in the bubbling phase:  MouseEvents           onClick           onDrag          onDoubleClick Keyboard Events                    onKeyDown                    onKeyPress                    onKeyUp Focus Events                  onFocus   onBlur To capture an event handler for the capture phase, append capture to the event name. For example, instead of using onClick, use onClickCapture to handle the click event.  Capture event example:                  Click me    Additional ExamplesExample1                       Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler   Example2       This example is along with HTML in a single file                                                            Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Adding Events: Below example is how you add an event. Highlighted in bold                      Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  Passing Arguments to Event HandlerThere are two ways arguments are passed to event handler  Arrow function                    this.handleClick(id,e)}>Click                onClick is the event                e is the event object                 id can be state or props or some data Bind method      Click  In this case event object is automatically passed In both methods e represents the react event and its passed after the ID as second argument,With an arrow function this event e is passed explicitly but with bind method its automatically passed.                                     Import React,{ Component } from “react”;                                         class TestApp extends Component {                                           state = {                                                       id: 2,                                                      Name: “TestApp Dummy”                                                };                                                             //arrow function                                                 handleClick = (id,e) => {                                                       console.log(id);                                                       console.log(e);                                                  };                                               handleArg = (e) => { this.handleClick(this.state.id,e);}                                                          render() {     return (                    TestApp,{this.state.name}            onClick={this.handleArg}>Display            );   }  }  The react event is an object and obtained from react. Instead of creating a separate function for passing argument, you can directly pass the anonymous arrow function as shown in the render function below:     render() {        return (                                                                                                       TestApp,{this.state.name}                                                {                           this.handleClick(this.state.id,e);                                                               }}>Display                                                                                                         );                                                 }                                            }    Output:   click on button  “TestApp Dummy “                   Let’s see only how bind method looks like in the render function    render() {                                         return (                                                                                                 TestApp,{this.state.name}                                                   Display                                                                                                       );                                                  }                                              } Output: this will display the h1 tag and when you click the button handleClick function gets invoked and the console will display id of the state object as shown above. Building a Practice to Thoroughly Understand Events This blog focuses on event handling, which in turn teaches about event handlers declared in JSX markup.This approach helps in tracking down the element mapped with events in an easy way.  We also learned how to handle multiple event handlers in a single element by using JSX attributes.we also learned about ways to bind event handler and  parameter values. Then we learned about synthetic events which are abstractions around native events. The best way you can retain this learning is by practicing more and tackling the complexities that may arise as you practice. You can find several tutorials on the internet or share your questions with us here. Happy learning! 
5355
Handling React Events - A Detailed Guide

Event handling essentially allows the user to inte... Read More

MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a collection or view and returns a cursor to those documents. There are two parameters in this formula: query and projection.Query – This is an optional parameter that specifies the criteria for selection. In simple terms, a query is what you want to search for within a collection.Projection – This is an optional parameter that specifies what should be returned if the query criteria are satisfied. In simple terms, it is a type of decision-making that is based on a set of criteria.MongoDB's Flexible SchemaA NoSQL database, which stands for "not only SQL," is a way of storing and retrieving data that is different from relational databases' traditional table structures (RDBMS).When storing large amounts of unstructured data with changing schemas, NoSQL databases are indeed a better option than RDBMS. Horizontal scaling properties of NoSQL databases allow them to store and process large amounts of data.These are intended for storing, retrieving, and managing document-oriented data, which is frequently stored in JSON format (JavaScript Object Notation). Document databases, unlike RDBMSs, have a flexible schema that is defined by the contents of the documents.MongoDB is one of the most widely used open-source NoSQL document databases. MongoDB is known as a 'schemaless' database because it does not impose a specific structure on documents in a collection.MongoDB is compatible with a number of popular programming languages. It also offers a high level of operational flexibility because it scales well horizontally, allowing data to be spread or 'sharded' across multiple commodity servers with the ability to add more servers as needed. MongoDB can be run on a variety of platforms, including developer laptops, private clouds, and public clouds.Querying documents using find()MongoDB queries are used to retrieve or fetch data from a MongoDB database. When running a query, you can use criteria or conditions to retrieve specific data from the database.The function db.collection is provided by MongoDB. find() is a function that retrieves documents from a MongoDB database.In MongoDB, the find method is used to retrieve a specific document from the MongoDB collection. In Mongo DB, there are a total of six methods for retrieving specific records.find()findAndModify()findOne()findOneAndDelete()findOneAndReplace()findOneAndUpdate()Syntax:find(query, projection)We can fetch a specific record using the Find method, which has two parameters. If these two parameters are omitted, the find method will return all of the documents in the MongoDB collection.Example:Consider an example of employees with the database of employee_id and employee_name and we will fetch the documents using find() method.First, create a database with the name “employees” with the following code:use employeesNow, create a collection “employee” with:db.createCollection("employee")In the next step we will insert the documents in the database:db.employee.insert([{employee_id: 101, employee_name: "Ishan"}, {employee_id: 102, employee_name: "Bhavesh"}, {employee_id: 103, employee_name: "Madan"}])Find all Documents:To get all the records in a collection, we need to use the find method with an empty parameter. In other words, when we need all the records, we will not use any parameters.db.employee.find()Output in Mongo ShellThe pretty() method can be used to display the results in a formatted manner.Syntax:db.COLLECTION_NAME.find().pretty()Let’s check our documents with pretty() method:Query FiltersWe will see examples of query operations using the db.collection.find() method in mongosh.We will use the employee collection in the employees database.db.employee.insert([{employee_id: 101, employee_name: "Ishan", age: 21, email_id: "ishanjain@gmail.com"}, {employee_id: 102, employee_name: "Bhavesh", age: 22, email_id: "bhaveshg@gmail.com"}, {employee_id: 103, employee_name: "Madan", age: 23, email_id: "madan@gmail.com"}])As we have seen earlier that to select all the documents in the database we pass an empty document as the query filter parameter to the find method.db.employee.find().pretty()Find the first document in a collection:db.employee.findOne()Find a document by ID:db.employee.findOne({_id : ObjectId("61d1ae0b56b92c20b423a5a7")})Find Documents that Match Query Criteriadb.employee.find({“age”: “22”})db.employee.find({"employee_name": "Madan"}).pretty()Sort Results by a Field:db.employee.find().sort({age: 1}).pretty()order by age, in ascending orderdb.employee.find().sort({age: -1}).pretty()order by age, in descending orderAND Conditions:A compound query can specify conditions for multiple fields in the documents in a collection. A logical AND conjunction connects the clauses of a compound query indirectly, allowing the query to select all documents in the collection that meet the specified conditions.In the following example, we will consider all the documents in the employee collection where employee_id equals 101 and age equals 21.db.employee.find({"employee_id": 101, "age": "21" }).pretty()Querying nested fieldsThe embedded or nested document feature in MongoDB is a useful feature. Embedded documents, also known as nested documents, are documents that contain other documents.You can simply embed a document inside another document in MongoDB. Documents are defined in the mongo shell using curly braces (), and field-value pairs are contained within these curly braces.Using curly braces, we can now embed or set another document inside these fields, which can include field-value pairs or another sub-document.Syntax:{ field: { field1: value1, field2: value2 } }Example:We have a database “nested” and in this database we have collection “nesteddoc”.The following documents will insert into the nesteddoc collection.db.nesteddoc.insertMany([ { "_id" : 1, "dept" : "A", "item" : { "sku" : "101", "color" : "red" }, "sizes" : [ "S", "M" ] }, { "_id" : 2, "dept" : "A", "item" : { "sku" : "102", "color" : "blue" }, "sizes" : [ "M", "L" ] }, { "_id" : 3, "dept" : "B", "item" : { "sku" : "103", "color" : "blue" }, "sizes" : "S" }, { "_id" : 4, "dept" : "A", "item" : { "sku" : "104", "color" : "black" }, "sizes" : [ "S" ] } ])Place the documents in the collection now. Also, take a look at the results:As a result, the nesteddoc collection contains four documents, each of which contains nested documents. The find() method can be used to access the collection's documents.db.nesteddoc.find()Specify Equality Condition:In this example, we will select the document from the nesteddoc query where dept equals “A”.db.nesteddoc.find({dept: "A"})Querying ArraysUse the query document {: } to specify an equality condition on an array, where is the exact array to match, including the order of the elements.The following query looks for all documents where the field tags value is an array with exactly two elements, "S" and "M," in the order specified:db.nesteddoc.find( { sizes: ["S", "M"] } )Use the $all operator to find an array that contains both the elements "S" and "M," regardless of order or other elements in the array:db.nested.find( { sizes: { $all: ["S", "M"] } } )Query an Array for an Element:The following example queries for all documents where size is an array that contains the string “S” as one of its elements:db.nesteddoc.find( { sizes: "S" } )Filter conditionsTo discuss the filter conditions, we will consider a situation that elaborates this. We will start by creating a collection with the name “products” and then add the documents to it.db.products.insertMany([ { _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }, { _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }, { _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }, { _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }, { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }])To check the documents, use db.products.find():$gt$gt selects documents with a field value greater than (or equal to) the specified value.db.products.find( { qty: { $gt: “20” } } )$gte:$gte finds documents in which a field's value is greater than or equal to (i.e. >=) a specified value (e.g. value.)db.products.find( { qty: { $gte: 20 } } )$lt:$lt selects documents whose field value is less than (or equal to) the specified value.db.products.find( { qty: { $lt: 25 } } )$lte:$lte selects documents in which the field's value is less than or equal to (i.e. =) the specified value.db.products.find( { qty: { $lte: 20 } } )Query an Array by Array Length:To find arrays with a specific number of elements, use the $size operator. For example, the following selects documents with two elements in the array.db.products.find( { "tags": {$size: 2} } )ProjectionIn MongoDB, projection refers to selecting only the data that is required rather than the entire document's data. If a document has five fields and you only want to show three of them, select only three of them.The find() method in MongoDB accepts a second optional parameter, which is a list of fields to retrieve, as explained in MongoDB Query Document. When you use the find() method in MongoDB, it displays all of a document's fields. To prevent this, create a list of fields with the values 1 or 0. The value 1 indicates that the field should be visible, while 0 indicates that it should be hidden.Syntax:db.COLLECTION_NAME.find({},{KEY:1})Example:We will consider the previous example of products collection. Run the below command on mongoshell to learn how projection works:db.products.find({},{"tags":1, _id:0})Keep in mind that the _id field is always displayed while executing the find() method; if you do not want this field to be displayed, set it to 0.Optimized FindingsTo retrieve a document from a MongoDB collection, use the Find method.Using the Find method, we can retrieve specific documents as well as the fields that we require. Other find methods can also be used to retrieve specific documents based on our needs.By inserting array elements into the query, we can retrieve specific elements or documents. To retrieve data for array elements from the collection in MongoDB, we can use multiple query operators.
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MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a c... Read More

Implementing MongoDb Map Reduce using Aggregation

Algorithms and applications in today's data-driven market collect data about people, processes, systems, and organisations 24 hours a day, seven days a week, resulting in massive amounts of data. The problem is figuring out how to process this massive amount of data efficiently without sacrificing valuable insights.What is Map Reduce? The MapReduce programming model comes to the rescue here. MapReduce, which was first used by Google to analyse its search results, has grown in popularity due to its ability to split and process terabytes of data in parallel, generating results faster. A (Key,value) pair is the basic unit of information in MapReduce. Before feeding the data to the MapReduce model, all types of structured and unstructured data must be translated to this basic unit. The MapReduce model, as the name implies, consists of two distinct routines: the Map-function and the Reduce-function.  MapReduce is a framework for handling parallelizable problems across huge files using a huge number of devices (nodes), which are collectively referred to as a cluster (if all nodes are on the same local network and use similar hardware) or a grid (if the nodes are shared across geographically and administratively distributed systems, and use more heterogeneous hardware).  When data stored in a filesystem (unstructured) or a database(structured) is processed, MapReduce can take advantage of data's locality, processing it close to where it's stored to reduce communication costs. Typically, a MapReduce framework (or system) consists of three operations: Map: Each worker node applies the map function to local data and saves the result to a temporary storage. Only one copy of the redundant input data is processed by a master node. Shuffle: worker nodes redistribute data based on output keys (produced by the map function), ensuring that all data associated with a single key is stored on the same worker node. Reduce: each group of output data is now processed in parallel by worker nodes, per key. This article will walk you through the Map-Reduce model's functionality step by step. Map Reduce in MongoDB The map-reduce operation has been deprecated since MongoDB 5.0. An aggregation pipeline outperforms a map-reduce operation in terms of performance and usability. Aggregation pipeline operators like $group, $merge, and others can be used to rewrite map-reduce operations. Starting with version 4.4, MongoDB provides the $accumulator and $function aggregation operators for map-reduce operations that require custom functionality. In JavaScript, use these operators to create custom aggregation expressions. The map and reduce functions are the two main functions here. As a result, the data is independently mapped and reduced in different spaces before being combined in the function and saved to the specified new collection. This mapReduce() function was designed to work with large data sets only. You can perform aggregation operations like max and avg on data using Map Reduce, which is similar to groupBy in SQL. It works independently and in parallel on data. Implementing Map Reduce with Mongosh (MongoDB Shell)  The db.collection.mapReduce() method in mongosh is a wrapper for the mapReduce command. The examples that follow make use of the db.collection.mapReduce(). Example: Create a collection ‘orders’ with these documents: db.orders.insertMany([     { _id: 1, cust_id: "Ishan Jain", ord_date: new Date("2021-11-01"), price: 25, items: [ { sku: "oranges", qty: 5, price: 2.5 }, { sku: "apples", qty: 5, price: 2.5 } ], status: "A" },     { _id: 2, cust_id: "Ishan Jain", ord_date: new Date("2021-11-08"), price: 70, items: [ { sku: "oranges", qty: 8, price: 2.5 }, { sku: "chocolates", qty: 5, price: 10 } ], status: "A" },     { _id: 3, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-08"), price: 50, items: [ { sku: "oranges", qty: 10, price: 2.5 }, { sku: "pears", qty: 10, price: 2.5 } ], status: "A" },     { _id: 4, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-18"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 5, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-19"), price: 50, items: [ { sku: "chocolates", qty: 5, price: 10 } ], status: "A"},     { _id: 6, cust_id: "Madan Parmar", ord_date: new Date("2021-11-19"), price: 35, items: [ { sku: "carrots", qty: 10, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 7, cust_id: "Madan Parmar", ord_date: new Date("2021-11-20"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 8, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 75, items: [ { sku: "chocolates", qty: 5, price: 10 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 9, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 55, items: [ { sku: "carrots", qty: 5, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 }, { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 10, cust_id: "Abhresh", ord_date: new Date("2021-11-23"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" }  ]) Apply a map-reduce operation to the orders collection to group them by cust_id, then add the prices for each cust_id: To process each input document, define the map function: this refers the document that the map-reduce operation is processing in the function. For each document, the function maps the price to the cust_id and outputs the cust_id and price. var mapFunction1 = function() {emit(this.cust_id, this.price);}; With the two arguments keyCustId and valuesPrices, define the corresponding reduce function: The elements of the valuesPrices array are the price values emitted by the map function, grouped by keyCustId. The valuesPrice array is reduced to the sum of its elements by this function. var reduceFunction1 = function(keyCustId, valuesPrices) {return Array.sum(valuesPrices);};Apply the mapFunction1 map function and the reduceFunction1 reduce function to all documents in the orders collection: db.orders.mapReduce(mapFunction1,reduceFunction1,{ out: "map_reduce_example" }) The results of this operation are saved in the map_reduce_example collection. If the map_reduce_example collection already exists, the operation will overwrite its contents with the map-reduce operation's results. Check the map_reduce_example collection to verify: db.map_reduce_example.find().sort( { _id: 1 } ) Aggregation Alternative:You can rewrite the map-reduce operation without defining custom functions by using the available aggregation pipeline operators: db.orders.aggregate([{$group: { _id:"$cust_id",value:{$sum: "$price" } } },{ $out: "agg_alternative_1" }]) Check the agg_alternative_1 collection to verify: db.agg_alternative_1.find().sort( { _id: 1 } )Implementing Map Reduce with Java Consider the collection car and insert the following documents in it. db.car.insert( [ {car_id:"c1",name:"Audi",color:"Black",cno:"H110",mfdcountry:"Germany",speed:72,price:11.25}, {car_id:"c2",name:"Polo",color:"White",cno:"H111",mfdcountry:"Japan",speed:65,price:8.5}, {car_id:"c3",name:"Alto",color:"Silver",cno:"H112",mfdcountry:"India",speed:53,price:4.5}, {car_id:"c4",name:"Santro",color:"Grey",cno:"H113",mfdcountry:"Sweden",speed:89,price:3.5} , {car_id:"c5",name:"Zen",color:"Blue",cno:"H114",mfdcountry:"Denmark",speed:94,price:6.5} ] ) You will get an output like this:  Let's now write the map reduce function on a collection of cars, grouping them by speed and classifying them as overspeed cars.  var speedmap = function (){  var criteria;  if ( this.speed > 70 ) {criteria = 'overspeed';emit(criteria,this.speed);}}; Based on the speed, this function classifies the vehicle as an overspeed vehicle. The term "this" refers to the current document that requires map reduction. var avgspeed_reducemap = function(key, speed) {       var total =0;       for (var i = 0; i 
7344
Implementing MongoDb Map Reduce using Aggregation

Algorithms and applications in today's data-driven... Read More