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Functions in JavaScript

In this blog we will learn more about the JavaScript functions. A function can be defined as a piece or block of code which is executed to perform a specific task. In other words, it is a group of reusable code which can be called anywhere in the program which makes the life of developers easy by eliminating the need of writing the same piece of code again and again. Functions help developers to divide larger programs into smaller chunks and also make them easily manageable. Functions allow us to define a block of code, give it a name and then execute it as many times as we want. We can define a JavaScript function using the keyword function.  Let’s explore JavaScript functions in detail. JavaScript Function Overview:- As in any other scripting and programming language, functions are one of the fundamental building blocks in JavaScript. A function is nothing but a series of statements that can be used to perform a task. In simple terms a function takes inputs and returns an output. In order to use a function, first we need to define it and then call the same. JavaScript provides us with many built in functions, for example  alert (“Welcome to Knowledge Hut”);  document.write(“Welcome to knowledge hut blogs”);  Math.sqr(3); Generally, functions always return a value. For example, a mathematical function performs calculations on the input data and returns an output. Other functions return true/false, or text. Some functions return no value but perform a side-effect; “write” is one such function whose side-effect is to send text to the HTML document.  JavaScript Function Syntax:- We use the keyword function to define a JavaScript function followed by the name of the function, and a list of parameters(can be empty also) enclosed in  parentheses (). function functionName (parameters) {  //your code    }  Ex: function helloMessage(){  alert(“welcome to Knowledge hut”); }   helloMessage(); function with parameters:- function printEmpDetails(empFirstName, empMiddleName, empLastName){  alert(“EmployeeDetails”+ empFirstName+””+ empMiddleName+””+ empLastName);  }  printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”);àOutputàEmployeeDetails Mohandas Karamchand Gandhi.  printEmpDetails(“William”,”Henry”,”Gates”); à OutputàEmployeeDetails William Henry Gates. functions with argument objects:- In JavaScript we can pass argument objects as default; argument object is just like an array object. function printEmpDetails(empFirstName, empMiddleName, empLastName){  alert(“EmployeeDetails”+ arguments[0]+””+ arguments[1]+””+ arguments[2]);  }  printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”);àOutputàEmployeeDetails Mohandas Karamchand Gandhi.  printEmpDetails(“William”,”Henry”,”Gates”); à OutputàEmployeeDetails William Henry Gates.  We can also iterate arguments as   function printEmpDetails () {      for(var i = 0; i < arguments.length; i++){   alert(arguments[i]);      }}    printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”); Function with return value:- By using a return statement in a function it will always return zero or 1 value. function addition(var1, var2) {      return var1 + var2;  };  var output = addition(5,15); à Output returns as 20, and assigns it to the variable output  function square(var1, var2) {      console.log( var1 * var2);  };  output = square(2,5); // Output does not return any value, since the variable has not been assigned to print the output. Function Expression:-In JavaScript we can also assign a function to the variable and then use that variable as the function. var sum = function addition(var1, var2) {      return var1 + var2;  };  var output = sum(5,15); à Valid way of calling function using variable   var output1 = addition (5,15); à Invalid way of calling function using variable    Anonymous Function:-  In JavaScript we can define a function without any name, this function without any name is known as an anonymous function.  var printMsg = function (){  alert("Welcome to Knowledge Hut!");  };  printMsg(); àoutput will be Welcome to Knowledge Hut.  Nested Functions:-  In JavaScript we can write more than one function. Nested functions will be in the scope of outer functions. Inner function will be able to access parameters and variables of the outer functions.  function printMsg(name){  function something() {  alert("Welcome " + name);  }return something();  }printMsg("Gandhi"); àOutput àWelcome Gandhi. JavaScript Function Invocation:- Whenever we want to execute the code that is written inside the function, we need to invoke that function. In JavaScript we can invoke functions in 4 different ways. As a function itself. As a method, using call() and apply() Using constructor. As a function itself:- This is the basic way of calling a function. function print(){  alert(“welcome to knowledge hut”);  }   print(); As a method:-  JavaScript provides us with predefined call() and apply() methods which can be used to invoke other functions. The major difference is apply() which takes arguments as an array, whereas call() takes arguments as one by one separately. The methods call() and apply() allow you to invoke the function, giving you a lot of control over how the function executes. var msg = function (str) {      alert("Student First Name " + this.var1 + ", Last Name" + str.var1);  };  var str1 = { var1: "APJ" };  var str2 = { var1: "Kalam" };  msg.call(str1, str2);  // => Student First Name APJ, Last Name Kalam  msg.call(str2, str1);  // => Student First Name Kalam, Last Name APJ  msg.apply(str1, [str2]); // => Student First Name APJ, Last Name Kalam  msg.apply(str2, [str1]); // => Student First Name Kalam, Last Name APJ The ‘this’ keyword In JavaScript, this is a special keyword which is used within methods to refer the object on which a method is being called. The value of ‘this’ can be determined using the following series of steps: If using call or apply to invoke the function, this will be set to the first argument passed to call/apply. If the first argument passed to call/apply is null or undefined, this will refer to the global object. If using bind to invoke the function, the this will be the first argument that was passed to bind at the time the function was created. If function is being invoked as a method of an object then this will refer to that object. If the function is invoked as a standalone function and not attached to any object, this will refer to the global object. Using constructor:- JavaScript allows us to create the function dynamically using the Function() constructor along with a new operator. Syntax:- new Function([arg1 [, arg2 [, ...argN]] ,] functionBody)  var sampleFunction = new Function('x', 'y', 'return x * y');   console.log(sampleFunction(5,5)); àOutput = 25    Adding a Method to a Constructor  function vName(firstName, lastName, age) {    this.visitorFirstName = firstName;    this.visitorlastName = lastName;    this.visitor = function() {return this.visitorFirstName + " " + this.visitorlastName;};  }  var visitorName = new vName("APJ","Kalam");  console.log("Welcome to Knowledge Hut” +visitorName.visitor());  outputà > Welcome to Knowledge Hut APJ Kalam Built In Constructors:- var a = new Date(); //new date object  var b = new String(); //new String object  var c = new Function(); //new function object JavaScript Function Parameters: -  When we invoke a function we can pass some parameters to the function. These parameters are used inside the function and we can pass as many parameters as required, separated by commas. function functionName(parameter1, parameter2,…, parameterN) {    // your code to be executed  } General rules for JavaScript parameters. We do not specify any datatype to the parameters. There is no type checking on the parameters passed. No restrictions on number of parameters passed. JavaScript also has argument object which is known as built-in object. Whenever a function is invoked the argument object which is an array of arguments can be used. var a = add(10,20,23,40);  function add() {    var x;    var add = 0;    for (x = 0; x < arguments.length; x++) {      add += arguments[x];    }    return add;  } How to close Functions:- JavaScript function closures are very important as they will control what is there and what is not there in the scope of the function. Closures are very important and frequently used in JavaScript for data privacy. Mainly it is used while handling events and call back functions. With the help of closures, inner functions have access to the enclosing/outer functions variables. function out() {     var y = 10;     function inn() {            var x = 20;            console.log(x+y);      }     return inn;  }   In the above example, we have defined two functions ‘in’ and ‘out’. An out function has been defined with a variable y and it returns the in (inner) function. Whereas ‘in’ function has its own variable x and accesses the ‘out’ variable y in the function body. Here the scope of the out variable is limited to out function; similarly the in function is limited to inner function. Now you may wonder, how does the in function access the y variable defined in the scope of the out function? This is where the ‘closure’ comes into picture in JavaScript. With closure, the inner function always gains access to the variables and parameters defined in the outer function, even after the outer function has returned. Closure is a function that always remembers the variables from the place where it is defined, regardless of where it is executed later. Conclusion:-In this blog we have discussed about JavaScript functions which are blocks of code and are executed repeatedly. Function helps us to avoid repeating the same code all over the place. You can use a function to wrap that code and reuse it. Functions can take zero or more than one argument. Since JavaScript is a loosely coupled language, we can pass any value to any of the data types and it is always optional to specify the parameter value while invoking the functions. Functions are the foundation of a good JavaScript programmer. Using them makes your programs better organized, clearer, more readable, and easier to understand when you wrap your code up in well-named functions for reuse. 

Functions in JavaScript

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Functions in JavaScript

In this blog we will learn more about the JavaScript functions. A function can be defined as a piece or block of code which is executed to perform a specific task. In other words, it is a group of reusable code which can be called anywhere in the program which makes the life of developers easy by eliminating the need of writing the same piece of code again and again. Functions help developers to divide larger programs into smaller chunks and also make them easily manageable. Functions allow us to define a block of code, give it a name and then execute it as many times as we want. We can define JavaScript function using the keyword function.  

Let’s explore JavaScript functions in detail. 

JavaScript Function Overview:- 

As in any other scripting and programming language, functions are one of the fundamental building blocks in JavaScript. A function is nothing but a series of statements that can be used to perform a task. In simple terms a function takes inputs and returns an output. In order to use a function, first we need to define it and then call the same. JavaScript provides us with many built in functions, for example  

alert (“Welcome to Knowledge Hut”); 
document.write(“Welcome to knowledge hut blogs”); 
Math.sqr(3); 

Generally, functions always return a value. For example, a mathematical function performs calculations on the input data and returns an output. Other functions return true/false, or text. Some functions return no value but perform a side-effect; write is one such function whose side-effect is to send text to the HTML document.  

JavaScript Function Syntax:- We use the keyword function to define a JavaScript function followed by the name of the function, and list of parameters(can be empty also) enclosed in  parentheses (). 

function functionName (parameters) { 
//your code    } 
Ex: function helloMessage(){ 
alert(“welcome to Knowledge hut”); } 
 helloMessage(); 

function with parameters:- 

function printEmpDetails(empFirstName, empMiddleName, empLastName){ 
alert(EmployeeDetails+ empFirstName+””+ empMiddleName+””+ empLastName); 
} 
printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”);àOutputàEmployeeDetails Mohandas Karamchand Gandhi. 
printEmpDetails(“William”,”Henry”,”Gates”); à OutputàEmployeeDetails William Henry Gates. 

functions with argument objects:- 

In JavaScript we can pass argument objects as default; argument object is just like an array object. 

function printEmpDetails(empFirstName, empMiddleName, empLastName){ 
alert(“EmployeeDetails”+ arguments[0]+””+ arguments[1]+””+ arguments[2]); 
} 
printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”);àOutputàEmployeeDetails Mohandas Karamchand Gandhi. 
printEmpDetails(“William”,”Henry”,”Gates”); à OutputàEmployeeDetails William Henry Gates. 
We can also iterate arguments as  
function printEmpDetails () { 
    for(var i = 0; i < arguments.lengthi++){ 
 alert(arguments[i]); 
    }} 
 
printEmpDetails(“Mohandas”,”Karamchand”,”Gandhi”); 

Function with return value:- 

By using a return statement in function it will always return zero or 1 value. 

function addition(var1, var2) { 
    return var1 + var2; 
};  var output = addition(5,15); à Output returns as 20, and assigns it to the variable output 
function square(var1, var2) { 
    console.log( var1 * var2); 
}; 
output = square(2,5); // Output does not return any value, since the variable has not been assigned to print the output. 

Function Expression:-

In JavaScript we can also assign a function to the variable and then use that variable as the function. 

var sum = function addition(var1, var2) { 
    return var1 + var2; 
}; 
var output = sum(5,15); à Valid way of calling function using variable  
var output1 = addition (5,15); à Invalid way of calling function using variable  
 Anonymous Function:-  In JavaScript we can define function without any name, this function without any name is known as an anonymous function. 
var printMsg = function (){ 
alert("Welcome to Knowledge Hut!"); 
}; 
printMsg(); àoutput will be Welcome to Knowledge Hut.  

Nested Functions:- 

 In JavaScript we can write more than one function. Nested functions will be in the scope of outer functions. Inner function will be able to access parameters and variables of the outer functions.  

function printMsg(name){ 
function something() { 
alert("Welcome " + name); 
}return something(); 
}printMsg("Gandhi"); àOutput àWelcome Gandhi. 

JavaScript Function Invocation:- 

Whenever we want to execute the code that is written inside the function, we need to invoke that function. In JavaScript we can invoke functions in 4 different ways. 

  • As a function itself. 
  • As a method, using call() and apply() 
  • Using constructor. 

As a function itself:-

 This is the basic way of calling a function. 

function print(){ 
alert(“welcome to knowledge hut”); 
 
print(); 

As a method:- 

 JavaScript provides us with predefined call() and apply() methods which can be used to invoke other functions. The major difference is apply(which takes arguments as an array, whereas call() takes arguments as one by one separately. The methods call() and apply() allow you to invoke the function, giving you a lot of control over how the function executes. 

var msg = function (str) { 
    alert("Student First Name " + this.var1 + ", Last Name" + str.var1); 
}; 
var str1 = { var1: "APJ" }; 
var str2 = { var1: "Kalam" }; 
msg.call(str1, str2);  // => Student First Name APJ, Last Name Kalam 
msg.call(str2, str1);  // => Student First Name Kalam, Last Name APJ 
msg.apply(str1, [str2]); // => Student First Name APJ, Last Name Kalam 
msg.apply(str2, [str1]); // => Student First Name Kalam, Last Name APJ 

The this keyword 

In JavaScript, this is a special keyword which is used within methods to refer the object on which a method is being called. The value of this can be determined using the following series of steps: 

  1. If using call or apply to invoke the function, this will be set to the first argument passed to call/apply. If the first argument passed to call/apply is null or undefined, this will refer to the global object. 
  2. If using bind to invoke the function, the this will be the first argument that was passed to bind at the time the function was created. 
  3. If function is being invoked as a method of an object then this will refer to that object. 
  4. If the function is invoked as a standalone function and not attached to any object, this will refer to the global object. 

Using constructor:- 

JavaScript allows us to create the function dynamically using the Function() constructor along with new operator. 

Syntax:- new Function([arg1 [, arg2 [, ...argN]] ,] functionBody) 
var sampleFunction = new Function('x', 'y', 'return x * y'); 
 console.log(sampleFunction(5,5)); àOutput = 25 
 
Adding a Method to a Constructor 
function vName(firstName, lastName, age) { 
  this.visitorFirstName = firstName; 
  this.visitorlastName = lastName; 
  this.visitor = function() {return this.visitorFirstName + " " + this.visitorlastName;}; 
} 
var visitorName = new vName("APJ","Kalam"); 
console.log("Welcome to Knowledge Hut” +visitorName.visitor()); 
outputà Welcome to Knowledge Hut APJ Kalam 

Built In Constructors:- 

var a = new Date(); //new date object 
var b = new String(); //new String object 
var c = new Function(); //new function object 

JavaScript Function Parameters: -  When we invoke a function we can pass some parameters to the function. These parameters are used inside the function and we can pass as many parameters as required, separated by commas. 

function functionName(parameter1, parameter2,…, parameterN) { 
  // your code to be executed 
} 

General rules for JavaScript parameters. 

  • We do not specify any datatype to the parameters. 
  • There is no type checking on the parameters passed. 
  • No restrictions on number of parameters passed. 

JavaScript also has argument object which is known as built-in object. Whenever a function is invoked the argument object which is an array of arguments can be used. 

var a = add(10,20,23,40); 
function add() { 
  var x; 
  var add = 0; 
  for (x = 0; x < arguments.length; x++) { 
    add += arguments[x]; 
  } 
  return add; 
} 

How to close Functions:- 

JavaScript function closures are very important as they will control what is there and what is not there in the scope of the function. Closures are very important and frequently used in JavaScript for data privacy. Mainly it is used while handling events and call back functions. With the help of closures, inner functions have access to the enclosing/outer functions variables. 

function out() { 
   var y = 10; 
   function inn() { 
          var x = 20;  
         console.log(x+y); 
    } 
   return inn; 
}   

In the above example, we have defined two functions in and out. An out function has been defined with a variable y and it returns the in (inner) function. Whereas in function has its own variable x and accesses the out variable y in the function body. Here the scope of the out variable is limited to out function; similarly the in function is limited to inner function. Now you may wonder, how does the in function access the y variable defined in the scope of the out function? This is where the closure comes into picture in JavaScript. With closure, the inner function always gains access to the variables and parameters defined in the outer function, even after the outer function has returned. Closure is a function that always remembers the variables from the place where it is defined, regardless of where it is executed later. 

Conclusion:-

In this blog we have discussed about JavaScript functions which are blocks of code and are executed repeatedly. Function helps us to avoid repeating the same code all over the place. You can use a function to wrap that code and reuse it. Functions can take zero or more than one argument. Since JavaScript is a loosely coupled language, we can pass any value to any of the data types and it is always optional to specify the parameter value while invoking the functions. Functions are the foundation of a good JavaScript programmer. Using them makes your programs better organized, clearer, more readable, and easier to understand when you wrap your code up in well-named functions for reuse. 

KnowledgeHut

KnowledgeHut

Author

KnowledgeHut is an outcome-focused global ed-tech company. We help organizations and professionals unlock excellence through skills development. We offer training solutions under the people and process, data science, full-stack development, cybersecurity, future technologies and digital transformation verticals.
Website : https://www.knowledgehut.com

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Let’s start with functional components by creating a  file as clickAppHandler.js.In this file let’s create a  functional component  as shown below                        Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  When onClick event triggers clickHandler function is called as shown below and when you click the button console will print the string “clicked” After this you need the add a component in the app component. In our code above you can see on click we pass the function as event handler and you will notice that we haven't added parentheses as it becomes a function, and we do not want that and we want handler to be a function not a function call. When a new component is rendered its event handler functions are added to the mapping maintained by the react.When the event is triggered and it hits and DOM object ,react maps the event to the handler, if it matches it calls the handler. 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! 
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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 
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Implementing MongoDb Map Reduce using Aggregation

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