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What Is a React Component? Write Your First React Component

Prior to ReactJS, Popular frameworks like Angular and Ember were focused more towards model/view/controller like patterns known as MVC. These frameworks tried to provide more functionalities and solve most front-end development problems. ReactJS, however, chose to follow a different path, focusing primarily on views and how we can efficiently render views, thus popularising Component-based Architecture in web development.  I wouldn’t go so far as to say the creators of ReactJS invented the Component-based Architecture pattern, but for sure they saw its benefits and helped popularize this pattern for web development.  But first, what is Component-based Architecture? Component-based Architecture In web development, final output is a fully functional UI. According to component-based architecture, we should see the complete UI as a combination of multiple smaller, reusable, composable, functional components and we should approach the problem in similar fashion. We should build multiple small, composable, functional components which can work together as a complete solution. What is component? So, we understand that in component-based architecture, we are supposed to build plenty of smaller components. But what is a component and how does it look like? The word "component" in software industry means a software package, a service or any module which encapsulate a set of related functionalities. In the context of UI development, a component means a UI element or a collection of UI elements which together provide some set of functionalities. A component could be as small and simple as a heading component which is builtwith single H1 tag and its text or it could be as complex as accordion or tabbed navigation. ReactJS also encourages web developers to build smaller, reusable, composable components.Benefits of using Component Architecture There are lot of discussions about the benefits of using Component Architecture. Some people feel that if we break down the UI in multiple components interacting with each other,then the code base will have too many components and will eventually make the project un-maintainable. But still, this architecture has following benefits: Reusability: It would always be great to avoid writing new code as much as we can and use the existing well tested functionalities. If we have collection of well tested reusable components then definitely, the overall output would be more robust. Faster Development: Reusing the same set of components will eventually take less time to develop a functionality. Composability: A big functionality is nothing but a collection of multiple small functionalities. Similarly, it is simpler to build a bigger functionality efficiently if we can compose multiple small components together. Less Maintainability: More reusability means less code and less code will always lead to less maintenance of code base.React Components React component is basically a collection of one or many UI elements which renders a complete functionality. It can show different behaviours based on different parameters. In ReactJS, these parameters are knownas state of component and props passed to component. So how does a React component looks like? A very simple components looks something like this:import React from "react";  function Greeting({ name }) {    return < h1 >{`Hello ${name}`}</h1>;  }  ReactDOM.render(<Greeting name="Gully Boy" />, document.getElementById("root"));Alright. But<Greeting /> is not recognised by web browsers so won't it throw an error? Above code uses JSX (Syntax extension for JavaScript) and there are tools which actually converts JSX into something understandable by JavaScript engines of web browsers.  JSX JSX is a library which allows us to write HTML (or XML) like syntax in JavaScript file. Then this code goes through a tool which converts it into something understandable by JavaScript engines. It allows us to write HTML in JS. It gives a syntax { <Any JS code >}, inside curly bracket, we can write JS code. But still how does the final React component looks like? Below code snippet is a final conversion of above Greeting component. import React from "react";  function Greeting({ name }) {    return React.createElement("h1", null, `Hello ${name}`);  }  ReactDOM.render(  React.createElement("Greeting", { name: "Gully Boy" }, null),  document.getElementById("root")  ); React.create Element is part of React API which actually defines a React element. This data structure will be used by React to render the DOM element later. So now, we understand the need of JSX and benefits of using JSX, We will be using React components using JSX in rest of the article.  Classification of React Components Based on Interactivity Before going into the classification, we need to understand what a state means in React component. state is information that a component itself maintains and which changes with time. The state affects the UI and/or the behaviour of the component. There are two types of components based on interactivity: Stateless Component: When a React componentdoesn't have its own state and it just renders based on the information passed to component (props) or a hardcoded information within the component then such components are called as stateless component. Such type of component doesn't provide any user interactivity with component. For Ex: above Greeting component <Greeting name="John Smith" /> is a stateless component as it just renders the information (in this case name props) passed to it. Stateful Component: There is other type of component which possesses state. This means based of the state behaviour of component changes. These types of components are known as stateful components. For Example a counter component below:import React, {useState} from 'react';  function Counter() {  const [value, setValue] = useState(0);  const increment = () => {  setValue(value + 1);  }  const decrement = () => {  setValue(value - 1);  }  return (  <div>  <button type="button" onClick={increment}>Decrement</button>  <span>{value}</span>  <button type="button" onClick={decrement}>Decrement</button>  </div>  );  }  export default Counter;Counter component has a state value which changes when user clicks on increment or decrement button. there is a span tag which displays the state value and it also gets updated based on user action. So this component is an example of stateful component. Based on Programming Methodology Counter component has a state value which changes when user clicks on increment or decrement button. there is a span tag which displays the state value and it also gets updated based on user action. So this component is an example of stateful component. Based on Programming Methodology There are two type of components based on Programming Methodology Class based Component: A React component can be JavaScript class which implements render method of ReactJS and extends React.Component class. Generally if a component has any state or need to implement any React lifecycle method then we create a class based component. Class based componentmust implement the render lifecycle method which returns the JSX. Stateless component can also be implemented as class based component. For example, Above <Greeting /> component can be implemented as class based component like below:import React from 'react';  class Greeting extends React.Component {  constructor(props) {  super(props);  }  render() {  return < h1 >{`Hello ${this.props.name}`}</h1>;  }  }  export default Greeting; Functional Component: A React component can be JavaScript function which returns JSX. Before Hooks which means before React v16.8, functional component was used only for building stateless component. Functional component takes props as argument and returns JSX. Writing functional code is always better in comparison to class based component because transpiled code for functional component is always shorter than class based component. For example, First implementation of <Greeting /> at the top is functional component. Based on Functionality There are two types of components categorised based on functionality. One typeimplementsbusiness logic and the other type is more inclined towards UI. Presentational Component: React components which decide the look and feel of UI are called presentational component. These components can be stateful or stateless components. State of these components are normally UI level state instead of application (business logic) state. These components are reusable and often used to have independent styles. For example: above Greeting component is a presentational component which can we be reused multiple places in application. Container Component: React component which mainly deals with application level state, rarely contains HTML tags and interact with API data are known as container component. This categorisation is basically separation of UI logic and application logic.Component and its lifecycle In React JS, class-based components are extended from React.Component class which provides several lifecycle methods to component. render is also a life cycle method which must be implemented in React component. Other lifecycle methods are optional and can be used to gain more control over the React component. As ReactJS has evolved with time, few lifecycle methods are no longer supported and will be removed from library in coming update. In this article, we will look into the lifecycle methods which are supported by library version 16.4+.  React lifecycle. To understand component's lifecycle methods and their invocation orders, we can divide all the methods in three broad category: Mounting, Updating, Unmounting Mounting: When component mounts then first of all constructor method (if present) is called. constructor method takes props as argument and pass props to the constructor method of React.Component class. Refer the example of class based component above. then getDerivedStateFromPropsstatic method will be invoked. getDerivedStateFromPropstakes two argument - props and state and returnsan object to update the state or returnsnull to update nothing. This lifecycle method is used when the state is dependson props. After getDerivedStateFromPropsmethod, render method is being called which returns the JSX. After render, componentDidMount is called which is the last method in mounting phase. We normally put API calls in componentDidMount lifecycle method. To summaries the mounting of component:constructor constructor(props) { super(props); // code here }getDerivedState From Props static getDerivedStateFromProps(props, state) {  // code here  // return updated state object or null  } render render() {  // code here  } componentDidMount componentDidMount() {  // code here  } Updating:  After a component is mounted, A React component is changed either when the state of component or props passed is changed. on update, first getDerivedStateFromPropsstatic method is called which returns the object to update the state or return null to update nothing. Then shouldComponentUpdate lifecycle method is called which takes two arguments (nextProps and nextState) and return a boolean. This method decides whether render method should be invoked or not. If this method returnstrue then render method will be invoked otherwise component will not get updated. This method is used to optimise the React component by reducing the number of re-renders. if shouldComponentUpdate return true (which is also the default return value of method) then render method will be invoked which updated the component based on updated state or new props. After render method getSnapshotBeforeUpdate method is called. this method is invoked right before the rendered output is committed to DOM. It takes two arguments prevProps and prevState. this method gives access to DOM before React is going to update it with recent changes. For example, you can access last scroll position before update (re-render). Value returned by getSnapshotBeforeUpdate will be passed to componentDidUpdate method which is the next lifecycle method invoked. componentDidUpdate is called right after update occurs. This method takes three arguments prevProps, prevState and snapshot. last argument snapshot will be the value returned from getSnapshotBeforeUpdate lifecycle method. To summarise update phase of component:getDerivedState From Props static getDerivedStateFromProps(props, state) {  // code here  // returns updated state object or null  } should Component Update shouldComponentUpdate(prevProps, prevState) {  // code here  // returns true to re-render the component otherwise return false.   // default is true  } render render() {  // code here  } getSnapshot Before Update getSnapshotBeforeUpdate(prevProps, prevState) {  // code here  } component Did Update componentDidUpdate(prevProps, preState, snapshot) {  // code here  } Unmounting: Last phase of React component is unmount. it invokes the last method of React component's lifecycle componentWillUnmount. this method is called before the component is unmounted and destroyed. Normally, clean up code is put in componentWillUnmount method. we should not call setState method here as component is never going to be re-rendered after unmount. To summarise:componentWillUnmount  componentWillUnmount() {  // code here  }Set up a React project As JavaScript has evolved and tooling has become an essential part of web development. There are many approaches to start a React project but the most popular one is create-react-app (popularly known as CRA). create-react-app is available as NPM module which means to use CRA, one must have NodeJS and NPM installed on machine. Create React project with CRA First check the NodeJS and NPM installation on machine by executing following command: node --version  npm --version  npx –version In case NodeJS is not install then download and install it from. Once it is confirmed that NodeJS and NPM is installed on system then execute following command to create a React project: npx create-react-app My First React Project NPX command comes with NPM. It is used to use any NPM without actually installing the NPM. above command will create a new React project with latest version of create-react-app in your current directory. Execute following command to run the project: cd My First React Project npm start This would start a local server at port number 3000 and open default React page in browser.Understanding project generated by CRAAbove image shows the files and folder generated by CRA. let us go through it and understand: package.json: This file contains NPM dependencies, NPM scripts, project name, version etc... public folder: All public assets will be present in public folder like index.html, image or logo etc... src folder: All our JS code including React components will be present in src folder. CRA configured our React project with all the needed tooling required for web development. It configured Webpack, dev server with hot reload ( on file save browser reloads the page to reflect latest change), Eslint, react-testing-library for unit testing React components etc... src/index.js is the starting point of our React project. Project also has one sample React component App.js. You can make changes to App.js and it will be reflected on browser. What type of React Component should I build in 2020 So far, we have learnt different ways of writing React component. Now it is time to choose which pattern should you choose for writing React components. As we have discussed in previous article that ReactJS is moving towards functional programming paradigm and with the introduction of hooks in version 16.8, Writing functional component fulfils most of our use cases.Let's create our first stateful functional component. We will be building a simple counter component. The final output looks like this: Counter component code: import React, { useState } from "react";  import "./Counter.css";  function Counter() {  const [count, setCount] = useState(0);  const increment = () => {  const updatedCount = count + 1;  setCount(updatedCount);  };  const decrement = () => {  const updatedCount = count - 1;  setCount(updatedCount);  };  return (  <div className="counterContainer">  <button type="button" onClick={decrement}>  - </button>  <span>{count}</span>  <button type="button" onClick={increment}>  +  </button>  </div>  );  } export default Counter; Styles are added in Counter.css .counterContainer {  background-color: #fff;  font-size: 24px;  font-weight: 500;  border: 1px solid #0874ce;  border-radius: 3px;  box-shadow: 1px 1px 4px 0 #0084ff;  }  .counterContainer button {  outline: none;  border: none;  width: 50px;  height: 40px;  background-color: transparent;  font-size: 24px;  font-weight: 500;  cursor: pointer;  }  .counterContainer span {  color: #0084ff;  } We have created a fully functional counter component which can be used anywhere in React application.

What Is a React Component? Write Your First React Component

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What Is a React Component? Write Your First React Component

Prior to ReactJS, Popular frameworks like Angular and Ember were focused more towards model/view/controller like patterns known as MVC. These frameworks tried to provide more functionalities and solve most front-end development problems. ReactJS, however, chose to follow a different path, focusing primarily on views and how we can efficiently render views, thus popularising Component-based Architecture in web development.  

I wouldn’t go so far as to say the creators of ReactJS invented the Component-based Architecture pattern, but for sure they saw its benefits and helped popularize this pattern for web development.  

But first, what is Component-based Architecture? 

Component-based Architecture 

In web development, final output is a fully functional UI. According to component-based architecture, we should see the complete UI as a combination of multiple smaller, reusable, composable, functional components and we should approach the problem in similar fashion. We should build multiple small, composable, functional components which can work together as a complete solution. 

What is component? 

So, we understand that in component-based architecture, we are supposed to build plenty of smaller components. But what is a component and how does it look like? 

The word "component" in software industry means a software package, a service or any module which encapsulate a set of related functionalities. In the context of UI development, a component means a UI element or a collection of UI elements which together provide some set of functionalities. A component could be as small and simple as a heading component which is builtwith single H1 tag and its text or it could be as complex as accordion or tabbed navigation. ReactJS also encourages web developers to build smaller, reusable, composable components.

Benefits of using Component Architecture 

There are lot of discussions about the benefits of using Component Architecture. Some people feel that if we break down the UI in multiple components interacting with each other,then the code base will have too many components and will eventually make the project un-maintainable. But still, this architecture has following benefits: 

  • Reusability: It would always be great to avoid writing new code as much as we can and use the existing well tested functionalities. If we have collection of well tested reusable components then definitely, the overall output would be more robust. 
  • Faster Development: Reusing the same set of components will eventually take less time to develop a functionality. 
  • Composability: A big functionality is nothing but a collection of multiple small functionalities. Similarly, it is simpler to build a bigger functionality efficiently if we can compose multiple small components together. 
  • Less Maintainability: More reusability means less code and less code will always lead to less maintenance of code base.

React Components 

React component is basically a collection of one or many UI elements which renders a complete functionality. It can show different behaviours based on different parameters. In ReactJS, these parameters are knownas state of component and props passed to component. So how does a React component looks like? 

A very simple components looks something like this:

import React from "react"; 
function Greeting({ name }) { 
  return < h1 >{`Hello ${name}`}</h1>; 
} 
ReactDOM.render(<Greeting name="Gully Boy" />, document.getElementById("root"));

Alright. But<Greeting /> is not recognised by web browsers so won't it throw an error? Above code uses JSX (Syntax extension for JavaScript) and there are tools which actually converts JSX into something understandable by JavaScript engines of web browsers.  

JSX 

JSX is a library which allows us to write HTML (or XML) like syntax in JavaScript file. Then this code goes through a tool which converts it into something understandable by JavaScript engines. It allows us to write HTML in JS. Igives a syntax { <Any JS code >}, inside curly bracket, we can write JS code. 

But still how does the final React component looks like? Below code snippet is a final conversion of above Greeting component. 

import React from "react"; 
function Greeting({ name }) { 
  return React.createElement("h1", null, `Hello ${name}`); 
} 
ReactDOM.render( 
React.createElement("Greeting", { name: "Gully Boy" }, null), 
document.getElementById("root") 
); 

React.create Element is part of React API which actually defines a React elementThis data structure will be used by React to render the DOM element later. So now, we understand the need of JSX and benefits of using JSX, We will be using React components using JSX in rest of the article.  

Classification of React Components 

Based on Interactivity 

Before going into the classification, we need to understand what a state means in React component. state is information that a component itself maintains and which changes with time. The state affects the UI and/or the behaviour of the component. 

There are two types of components based on interactivity: 

  • Stateless Component: When a React componentdoesn't have its own state and it just renders based on the information passed to component (props) or a hardcoded information within the component then such components are called as stateless component. Such type of component doesn't provide any user interactivity with component. For Ex: above Greeting component <Greeting name="John Smith" /> is a stateless component as it just renders the information (in this case name props) passed to it. 

  • Stateful Component: There is other type of component which possesses state. This means based of the state behaviour of component changes. These types of components are known as stateful components. For Example a counter component below:

import React, {useState} from 'react'; 
function Counter() { 
const [value, setValue] = useState(0); 
const increment = () => { 
setValue(value + 1); 
} 
const decrement = () => { 
setValue(value - 1); 
} 
return ( 
<div> 
<button type="button" onClick={increment}>Decrement</button> 
<span>{value}</span> 
<button type="button" onClick={decrement}>Decrement</button> 
</div> 
); 
} 
export default Counter;

Counter component has a state value which changes when user clicks on increment or decrement button. there is a span tag which displays the state value and it also gets updated based on user action. So this component is an example of stateful component. 

Based on Programming Methodology 

Counter component has a state value which changes when user clicks on increment or decrement button. there is a span tag which displays the state value and it also gets updated based on user action. So this component is an example of stateful component. 

Based on Programming Methodology 

There are two type of components based on Programming Methodology 

  • Class based Component: A React component can be JavaScript class which implements render method of ReactJS and extends React.Component class. Generally if a component has any state or need to implement any React lifecycle method then we create a class based component. Class based componentmust implement the render lifecycle method which returns the JSX. Stateless component can also be implemented as class based component. For example, Above <Greeting /> component can be implemented as class based component like below:

import React from 'react'; 
class Greeting extends React.Component { 
constructor(props) { 
super(props); 
} 
render() { 
return < h1 >{`Hello ${this.props.name}`}</h1>; 
} 
} 
export default Greeting; 
  • Functional Component: A React component can be JavaScript function which returns JSX. Before Hooks which means before React v16.8, functional component was used only for building stateless component. Functional component takes props as argument and returns JSX. Writing functional code is always better in comparison to class based component because transpiled code for functional component is always shorter than class based component. For example, First implementation of <Greeting /> at the top is functional component. 

Based on Functionality 

There are two types of components categorised based on functionality. One typeimplementsbusiness logic and the other type is more inclined towards UI. 

  • Presentational Component: React components which decide the look and feel of UI are called presentational component. These components can be stateful or stateless components. State of these components are normally UI level state instead of application (business logic) state. These components are reusable and often used to have independent styles. For example: above Greeting component is a presentational component which can we be reused multiple places in application. 

  • Container Component: React component which mainly deals with application level state, rarely contains HTML tags and interact with API data are known as container component. This categorisation is basically separation of UI logic and application logic.

Component and its lifecycle 

In React JS, class-based components are extended from React.Component class which provides several lifecycle methods to component. render is also a life cycle method which must be implemented in React component. Other lifecycle methods are optional and can be used to gain more control over the React component. As ReactJS has evolved with time, few lifecycle methods are no longer supported and will be removed from library in coming update. In this article, we will look into the lifecycle methods which are supported by library version 16.4+.  

React lifecycle

React lifecycle. 

To understand component's lifecycle methods and their invocation orders, we can divide all the methods in three broad category: Mounting, Updating, Unmounting 

  • Mounting: When component mounts then first of all constructor method (if present) is called. constructor method takes props as argument and pass props to the constructor method of React.Component class. Refer the example of class based component above. then getDerivedStateFromPropsstatic method will be invoked. getDerivedStateFromPropstakes two argument - props and state and returnsan object to update the state or returnsnull to update nothing. This lifecycle method is used when the state is dependson props. After getDerivedStateFromPropsmethod, render method is being called which returns the JSX. After render, componentDidMount is called which is the last method in mounting phase. We normally put API calls in componentDidMount lifecycle method. To summaries the mounting of component:

  • constructor 

constructor(props) { 
super(props); 
// code here 
}
  • getDerivedState From Props 

static getDerivedStateFromProps(props, state) { 
// code here 
// return updated state object or null 
} 
  • render 

render() { 
// code here 
} 
  • componentDidMount 

componentDidMount() { 
// code here 
} 
  • Updating:  After a component is mounted, A React component is changed either when the state of component or props passed is changed. on update, first getDerivedStateFromPropsstatic method is called which returns the object to update the state or return null to update nothing. Then shouldComponentUpdate lifecycle method is called which takes two arguments (nextProps and nextState) and return a boolean. This method decides whether render method should be invoked or not. If this method returnstrue then render method will be invoked otherwise component will not get updated. This method is used to optimise the React component by reducing the number of re-renders. if shouldComponentUpdate return true (which is also the default return value of method) then render method will be invoked which updated the component based on updated state or new props. After render method getSnapshotBeforeUpdate method is called. this method is invoked right before the rendered output is committed to DOM. It takes two arguments prevProps and prevState. this method gives access to DOM before React is going to update it with recent changes. For example, you can access last scroll position before update (re-render). Value returned by getSnapshotBeforeUpdate will be passed to componentDidUpdate method which is the next lifecycle method invoked. componentDidUpdate is called right after update occurs. This method takes three arguments prevProps, prevState and snapshot. last argument snapshot will be the value returned from getSnapshotBeforeUpdate lifecycle method. To summarise update phase of component:
  • getDerivedState From Props 

static getDerivedStateFromProps(props, state) { 
// code here 
// returns updated state object or null 
} 
  • should Component Update 

shouldComponentUpdate(prevPropsprevState) { 
// code here 
// returns true to re-render the component otherwise return false.  
// default is true 
} 
  • render 

render() { 
// code here 
} 
  • getSnapshot Before Update 

getSnapshotBeforeUpdate(prevPropsprevState) { 
// code here 
} 
  • component Did Update 

componentDidUpdate(prevPropspreState, snapshot) { 
// code here 
} 
  • Unmounting: Last phase of React component is unmount. it invokes the last method of React component's lifecycle componentWillUnmount. this method is called before the component is unmounted and destroyed. Normally, clean up code is put in componentWillUnmount method. we should not call setState method here as component is never going to be re-rendered after unmount. To summarise:

componentWillUnmount 
componentWillUnmount() { 
// code here 
}

Set up a React project 

As JavaScript has evolved and tooling has become an essential part of web development. There are many approaches to start a React project but the most popular one is create-react-app (popularly known as CRA). create-react-app is available as NPM module which means to use CRA, one must have NodeJS and NPM installed on machine. 

Create React project with CRA 

First check the NodeJS and NPM installation on machine by executing following command: 

node --version 
npm --version 
npx –version 

In case NodeJS is not install then download and install it from. 

Once it is confirmed that NodeJS and NPM is installed on system then execute following command to create a React project: 

npx create-react-app My First React Project 

NPX command comes with NPM. It is used to use any NPM without actually installing the NPM. above command will create a new React project with latest version of create-react-app in your current directory. Execute following command to run the project: 

cd My First React Project 

npm start 

This would start a local server at port number 3000 and open default React page in browser.

React page in browser

Understanding project generated by CRA project generated by CRA

Above image shows the files and folder generated by CRA. let us go through it and understand: 

  • package.json: This file contains NPM dependencies, NPM scripts, project name, version etc... 
  • public folder: All public assets will be present in public folder like index.html, image or logo etc... 
  • src folder: All our JS code including React components will be present in src folder. 

CRA configured our React project with all the needed tooling required for web development. It configured Webpackdev server with hot reload ( on file save browser reloads the page to reflect latest change), Eslintreact-testing-library for unit testing React components etc... 

src/index.js is the starting point of our React project. Project also has one sample React component App.js. You can make changes to App.js and it will be reflected on browser. 

What type of React Component should I build in 2020 

So far, we have learnt different ways of writing React component. Now it is time to choose which pattern should you choose for writing React components. As we have discussed in previous article that ReactJS is moving towards functional programming paradigm and with the introduction of hooks in version 16.8, Writing functional component fulfils most of our use cases.

Let's create our first stateful functional component. We will be building a simple counter component. The final output looks like this: 

Counter component code: 

import React, { useState } from "react"; 
import "./Counter.css"; 
function Counter() { 
const [count, setCount] = useState(0); 
const increment = () => { 
const updatedCount = count + 1; 
setCount(updatedCount); 
}; 
const decrement = () => { 
const updatedCount = count - 1; 
setCount(updatedCount); 
}; 
return ( 
<div className="counterContainer"> 
<button type="button" onClick={decrement}> 
-
</button> 
<span>{count}</span> 
<button type="button" onClick={increment}> 
+ 
</button> 
</div> 
); 
} 

export default Counter; 

Styles are added in Counter.css 

.counterContainer { 
background-color: #fff; 
font-size: 24px; 
font-weight: 500; 
border: 1px solid #0874ce; 
border-radius: 3px; 
box-shadow: 1px 1px 4px 0 #0084ff; 
} 
.counterContainer button { 
outline: none; 
border: none; 
width: 50px; 
height: 40px; 
background-color: transparent; 
font-size: 24px; 
font-weight: 500; 
cursor: pointer; 
} 
.counterContainer span { 
color: #0084ff; 
} 

We have created a fully functional counter component which can be used anywhere in React application.

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Handling React Events - A Detailed Guide

Event handling essentially allows the user to interact with a webpage and do something specific when a certain event like a click or a hover happens. When the user interacts with the application, events are fired, for example, mouseover, key press, change event, and so on. The application must handle events and execute the code. In short, events are the actions to which javascript can respond.   The actions to which javascript can respond are called events. Handling events with react is  very similar to handling events in DOM elements. Below are some general events that you would see in and out when dealing with react based websites:  Clicking an element  Submitting a form Scrolling page Hovering an element  Loading a webpage Input field change User stroking a key Image loading Naming Events in React Handling events with react is very similar to handling events in DOM elements, although there are some syntactic differences.   React events are written in camelCase.   A function is passed as the event handler rather than string. The way to write events in html / DOM is below:        click me onclick is written in lower case in html as shown above and what action to take when this onclick event triggers is taken care of by handleClick.In React, events are named using camel case and you pass a function as event handler as shown below:  Like in a functional component, event is written like below:       click me   In class based component ,event is written like below        click me Defining Events:Events are normally used in combination with functions, and the function is not executed until the event occurs, and the combination of event, HTML element, and javascript function is called binding which means to map all three. Generic syntax is:      Example:  Create a button element and what happens when onClick event triggered is driven by the function which is func() shown below     click me Let’s see some of the event attributes:   onmouseover : The mouse is moved over an element onmouseup : The mouse button is released onmouseout : The mouse  is moved off an element onmousemove: The mouse is moved Onmousedown: mouse button is pressed  onload : A image is done loading onunload: Existing the page  onblur : Losing Focus  on element  onchange : Content of a field changes onclick: Clicking an object  ondblclick: double clicking an object  onfocus element getting a focus  Onkeydown: pushing a keyboard key Onkeyup: keyboard key is released Onkeypress: keyboard key is pressed  Onselect: text is selected These are some examples of events:                                         Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Synthetic Events When you specify an event in JSX, you are not directly dealing with regular DOM events, you are dealing with a react event type called a synthetic event.It's a simple wrapper for native event instances and every synthetic event created needs to be garbage-collected which can be resource intensive in terms of CPU. The synthetic event object has properties mentioned below:  Boolean isTrusted  DOMEvent nativeEvent number timeStamp   void preventDefault() number eventPhase Synthetic events provide an interface and reduce browser inconsistencies and the event contains required information for its propagation to work. Synthetic event is reused for performance reasons in the browser, A synthetic event is a cross-browser wrapper around the browser’s native event it has the same interface as the native event. Synthetic events are delegated to the document node. Therefore native events are triggered first and the events bubble up to document, after which the synthetic events are triggered. The synthetic event object will be reused and all the properties will be nullified after the event callback has been invoked and this is for performance reasons.The workflow of synthetic event in react is:    Element ---- > Event ---- > synthetic event  ---- > handler(e)                                |                                                      |                                |  _______  Component ________|  umber timeStamp The Basics of React Event Handling Let’s explore how to handle events in react and we will showcase the click event and how it holds good for other types of events. 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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