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What is query string in Node.js?

In this article, we will look into query string module in Node.js, and understand a few methods to deal with query string. The available methods can be used to convert query string into a JSON object and convert a JSON object into query string.What is Query StringA query string according to Wikipedia is a part of the uniform resource locator (URL), that assigns values to specified parameters. In plain English it is the string after the ? in a url. Some url examples are shown below.https://example.com/over/there?name=ferret https://example.com/path/to/page?name=ferret&color=purpleThe query string in first case is name=ferret and in second case is name=ferret&color=purpleNode.js Query string moduleNow, the Node.js query string module provides methods for parsing and formatting URL query strings. The query string module can be accessed using the below –const querystring = require(‘querystring’)We will now look into the below six methods in the next section.querystring.decode()querystring.encode()querystring.escape(str)querystring.parse(str[, sep[, eq[, options]]])querystring.stringify(obj[, sep[, eq[, options]]])querystring.unescape(str)Query String methods with descriptionLet us look into a real example to understand the important Query string methods. Let us setup a basic Node application by giving the command npm init -y in terminal, inside a folder. I had created an empty NodeJS folder for the same.$ npm init -y Wrote to D:\NodeJS\package.json: {   "name": "NodeJS",   "version": "1.0.0",   "description": "",   "main": "index.js",   "scripts": {     "test": "echo \"Error: no test specified\" && exit 1"   },   "keywords": [],   "author": "",   "license": "ISC" }The above commands create a basic package.json file, which is the basis of any Node.js project. We are using the -y option, so that we don’t have to enter the details manually.Next, open the folder in a code editor which is VSCode in my case. Here, I have created a file querystring.js and the first line contains the importing of querystring module.querystring.parse() MethodThe querystring.parse() method is used to parse the URL query string into an object that contains the key value pair. The object which we get is not a JavaScript object, so we cannot use Object methods like obj.toString, or obj.hasOwnProperty().The latest UTF-8 encoding format is assumed unless we specify a different encoding format. But we should stick to UTF-8 encoding as it is the standard and contains all international characters like the Chinese characters and the hindi characters. After that also if we need alternative character encoding, then the decodeURIComponent option should be used.The syntax for the method is below.querystring.parse( str[, sep[, eq[, options]]]) )As seen from the above syntax the method accepts four parameters, and they are described below.str: This is the only required string field and it specifies the query string that has to be parsed.sep: It is an optional string field, which if given specifies the substring used to delimit the key and value pairs in the query string. The default value which is generally used is “&”.eq: It is an optional string field that specifies the substring used to delimit keys and values in the query string. The default value which is generally used is “=”.options: It is an optional object field which is used to modify the behaviour of the method. It can have the following parameters:decodeURIComponent: It is a function that would be used to specify the encoding format in the query string. The default value is querystring.unescape(), about which we will learn later.maxKeys: It is the number which specifies the maximum number of keys that should be parsed. A value of “0” would remove all the counting limits, and it can parse any number of keys. The default value is set at “1000”.The below example shows the various options used in querystring.parse() method. Add the below code in querystring.js file, which we created earlier.// Import the querystring module const querystring = require("querystring");   // Specify the URL query string to be parsed   let urlQueryString = "name=nabendu&units=kgs&units=pounds&login=false";   // Use the parse() method on the string   let parsedObj = querystring.parse(urlQueryString);   console.log("Parsed Query 1:", parsedObj);   // Use the parse() method on the string with sep as `&&` and eq as `-` urlQueryString = "name-nabendu&&units-kgs&&units-pounds&&login-true"; parsedObj = querystring.parse(urlQueryString, "&&", "-");   console.log("\nParsed Query 2:", parsedObj); // Specify a new URL query string to be parsed   urlQueryString = "type=admin&articles=java&articles=javascript&articles=kotlin&access=true"; // Use the parse() method on the string with maxKeys set to 1   parsedObj = querystring.parse(urlQueryString, "&", "=", { maxKeys: 1 });   console.log("\nParsed Query 3:", parsedObj);   // Use the parse() method on the string with maxKeys set to 2   parsedObj =  querystring.parse(urlQueryString, "&", "=", { maxKeys: 2 });   console.log("\nParsed Query 4:", parsedObj); // Use the parse() method on the string with maxKeys set to 0 (no limits) parsedObj = querystring.parse(urlQueryString, "&", "=", { maxKeys: 0 }); console.log("\nParsed Query 5:", parsedObj);Now, run the command node querystring.js from the Integrated terminal in VSCode or any terminal. Note that you need to be inside the folder NodeJS, which we had created earlier. The output of the same will be below.Parsed Query 1: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'false' } Parsed Query 2: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'true' } Parsed Query 3: [Object: null prototype] { type: 'admin' } Parsed Query 4: [Object: null prototype] { type: 'admin', articles: 'java' } Parsed Query 5: [Object: null prototype] {   type: 'admin',   articles: [ 'java', 'javascript', 'kotlin' ],   access: 'true' }querystring.stringify() MethodThe querystring.stringify() method is used to produce a query string from a given object, which contains a key value pair. It is exactly the opposite of querystring.parse() Method.It can be used to convert the string, numbers and Boolean values for the key. You can also use an array of string, numbers or Boolean as values. This method of changing an object to query string is called serialized.The latest UTF-8 encoding format is assumed unless we specify a different encoding format. But we should stick to UTF-8 encoding as it is the standard and contains all international characters like the Chinese characters and the Hindi characters. If we still need an alternative character encoding, then the decodeURIComponent option should be used.Syntax for the method is below.querystring. stringify( obj[, sep[, eq[, options]]]) )As from the above syntax the method accepts four parameters, and they are described below.obj: This is the only required object field and it specifies the object that has to be serialized.sep: It is an optional string field, which if given specifies the substring used to delimit the key and value pairs in the query string. The default value which is generally used is “&”.eq: It is an optional string field that specifies the substring used to delimit keys and values in the query string. The default value which is generally used is “=”.options: It is an optional object field which is used to modify the behaviour of the method. It can have the following parameters:decodeURIComponent: It is a function that would be used to specify the encoding format in the query string. The default value is querystring.escape(), about which we will learn later.The below example shows the various options used in querystring.stringify() method. Add the below code in querystring.js file, which we created earlier.// Import the querystring module   const querystring = require("querystring");   // Specify the object that needed to be serialized   let obj = {   name: "nabendu",   access: true,   role: ["developer", "architect", "manager"],   };   // Use the stringify() method on the object   let queryString = querystring.stringify(obj);   console.log("Query String 1:", queryString);   obj = {       name: "Parag",       access: false,       role: ["editor", "HR"],   };   // Use the stringify() method on the object with sep as `, ` and eq as `:` queryString = querystring.stringify(obj, ", ", ":");   console.log("Query String 2:", queryString);   // Use the stringify() method on the object with sep as `&&&` and eq as `==`   queryString = querystring.stringify(obj, "&&&", "==");   console.log("\nQuery String 3:", queryString);Now, run the command node querystring.js from the Integrated terminal in VSCode or any terminal. Note that you need to be inside the folder NodeJS, which we had created earlier. The output of the same will be below.Query String 1: name=nabendu&access=true&role=developer&role=architect&role=managerQuery String 2: name:Parag, access:false, role:editor, role:HR        Query String 3: name==Parag&&&access==false&&&role==editor&&&role==HRquerystring.decode() MethodThe querystring.decode() method is nothing but an alias for querystring.parse() method. In our parse example, we can use it. So, add the below code in querystring.js file, which we created earlier.// Import the querystring module const querystring = require("querystring");   // Specify the URL query string to be parsed   let urlQueryString = "name=nabendu&units=kgs&units=pounds&login=false";   // Use the parse() method on the string   let parsedObj = querystring.decode(urlQueryString); console.log("Parsed Query 1:", parsedObj);As earlier, run the command node querystring.js from a terminal. And the output will be same as that with querystring.parse() method.Parsed Query 1: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'false' }querystring.encode() MethodThe querystring.encode() method is nothing but an alias for querystring.stringify() method. In our stringify example, we can use it. So, add the below code in querystring.js file, which we created earlier.// Import the querystring module   const querystring = require("querystring");   // Specify the object that needed to be serialized let obj = { name: "nabendu", access: true,   role: ["developer", "architect", "manager"],   };   // Use the stringify() method on the object   let queryString = querystring.encode(obj);   console.log("Query String 1:", queryString);As earlier run the command node querystring.js from a terminal. And the output will be same as that with querystring.stringify() method.Query String 1: name=nabendu&access=true&role=developer&role=architect&role=managerquerystring.escape(str) MethodThe querystring.escape() method is used by querystring.stringify() method and is generally not used directly.querystring.unescape(str) MethodThe querystring.unescape() method is used by querystring.parse() method and is generally not used directly. SummaryIn this article we learnt about the useful query string module in Node.js, which is mainly used to parse URL query strings into Object format and also to change an object to URL query strings.

What is query string in Node.js?

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What is query string in Node.js?

In this article, we will look into query string module in Node.js, and understand a few methods to deal with query string. The available methods can be used to convert query string into a JSON object and convert a JSON object into query string.

What is Query String

A query string according to Wikipedia is a part of the uniform resource locator (URL), that assigns values to specified parameters. In plain English it is the string after the ? in a url. Some url examples are shown below.

https://example.com/over/there?name=ferret
https://example.com/path/to/page?name=ferret&color=purple

The query string in first case is name=ferret and in second case is name=ferret&color=purple

Node.js Query string module

Now, the Node.js query string module provides methods for parsing and formatting URL query strings. The query string module can be accessed using the below –

const querystring = require(‘querystring’)

We will now look into the below six methods in the next section.

  • querystring.decode()
  • querystring.encode()
  • querystring.escape(str)
  • querystring.parse(str[, sep[, eq[, options]]])
  • querystring.stringify(obj[, sep[, eq[, options]]])
  • querystring.unescape(str)

Query String methods with description

Let us look into a real example to understand the important Query string methods. Let us setup a basic Node application by giving the command npm init -y in terminal, inside a folder. I had created an empty NodeJS folder for the same.

$ npm init -y
Wrote to D:\NodeJS\package.json:
{
  "name": "NodeJS",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

The above commands create a basic package.json file, which is the basis of any Node.js project. We are using the -y option, so that we don’t have to enter the details manually.

Next, open the folder in a code editor which is VSCode in my case. Here, I have created a file querystring.js and the first line contains the importing of querystring module.

querystring.parse() Method

The querystring.parse() method is used to parse the URL query string into an object that contains the key value pair. The object which we get is not a JavaScript object, so we cannot use Object methods like obj.toString, or obj.hasOwnProperty().

The latest UTF-8 encoding format is assumed unless we specify a different encoding format. But we should stick to UTF-8 encoding as it is the standard and contains all international characters like the Chinese characters and the hindi characters. After that also if we need alternative character encoding, then the decodeURIComponent option should be used.

The syntax for the method is below.

querystring.parse( str[, sep[, eq[, options]]]) )

As seen from the above syntax the method accepts four parameters, and they are described below.

  • str: This is the only required string field and it specifies the query string that has to be parsed.
  • sep: It is an optional string field, which if given specifies the substring used to delimit the key and value pairs in the query string. The default value which is generally used is “&”.
  • eq: It is an optional string field that specifies the substring used to delimit keys and values in the query string. The default value which is generally used is “=”.
  • options: It is an optional object field which is used to modify the behaviour of the method. It can have the following parameters:
    • decodeURIComponent: It is a function that would be used to specify the encoding format in the query string. The default value is querystring.unescape(), about which we will learn later.
    • maxKeys: It is the number which specifies the maximum number of keys that should be parsed. A value of “0” would remove all the counting limits, and it can parse any number of keys. The default value is set at “1000”.

The below example shows the various options used in querystring.parse() method. Add the below code in querystring.js file, which we created earlier.

// Import the querystring module 
const querystring = require("querystring");  
// Specify the URL query string to be parsed  
let urlQueryString = "name=nabendu&units=kgs&units=pounds&login=false";  
// Use the parse() method on the string  
let parsedObj = querystring.parse(urlQueryString);  
console.log("Parsed Query 1:", parsedObj);  
// Use the parse() method on the string with sep as `&&` and eq as `-` 
urlQueryString = "name-nabendu&&units-kgs&&units-pounds&&login-true"; 
parsedObj = querystring.parse(urlQueryString, "&&", "-");  
console.log("\nParsed Query 2:", parsedObj);
// Specify a new URL query string to be parsed  
urlQueryString = "type=admin&articles=java&articles=javascript&articles=kotlin&access=true";
// Use the parse() method on the string with maxKeys set to 1  
parsedObj = querystring.parse(urlQueryString, "&", "=", { maxKeys: 1 });  
console.log("\nParsed Query 3:", parsedObj);  
// Use the parse() method on the string with maxKeys set to 2  
parsedObj =  querystring.parse(urlQueryString, "&", "=", { maxKeys: 2 });  
console.log("\nParsed Query 4:", parsedObj); 
// Use the parse() method on the string with maxKeys set to 0 (no limits) 
parsedObj = querystring.parse(urlQueryString, "&", "=", { maxKeys: 0 }); 
console.log("\nParsed Query 5:", parsedObj);

Now, run the command node querystring.js from the Integrated terminal in VSCode or any terminal. Note that you need to be inside the folder NodeJS, which we had created earlier. The output of the same will be below.

Parsed Query 1: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'false' }
Parsed Query 2: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'true' }
Parsed Query 3: [Object: null prototype] { type: 'admin' }
Parsed Query 4: [Object: null prototype] { type: 'admin', articles: 'java' }
Parsed Query 5: [Object: null prototype] {
  type: 'admin',
  articles: [ 'java', 'javascript', 'kotlin' ],
  access: 'true' }

querystring.stringify() Method

The querystring.stringify() method is used to produce a query string from a given object, which contains a key value pair. It is exactly the opposite of querystring.parse() Method.

It can be used to convert the string, numbers and Boolean values for the key. You can also use an array of string, numbers or Boolean as values. This method of changing an object to query string is called serialized.

The latest UTF-8 encoding format is assumed unless we specify a different encoding format. But we should stick to UTF-8 encoding as it is the standard and contains all international characters like the Chinese characters and the Hindi characters. If we still need an alternative character encoding, then the decodeURIComponent option should be used.

Syntax for the method is below.

querystring. stringify( obj[, sep[, eq[, options]]]) )

As from the above syntax the method accepts four parameters, and they are described below.

  • obj: This is the only required object field and it specifies the object that has to be serialized.
  • sep: It is an optional string field, which if given specifies the substring used to delimit the key and value pairs in the query string. The default value which is generally used is “&”.
  • eq: It is an optional string field that specifies the substring used to delimit keys and values in the query string. The default value which is generally used is “=”.
  • options: It is an optional object field which is used to modify the behaviour of the method. It can have the following parameters:
    • decodeURIComponent: It is a function that would be used to specify the encoding format in the query string. The default value is querystring.escape(), about which we will learn later.

The below example shows the various options used in querystring.stringify() method. Add the below code in querystring.js file, which we created earlier.

// Import the querystring module  
const querystring = require("querystring");  
// Specify the object that needed to be serialized  
let obj = {  
name: "nabendu",  
access: true,  
role: ["developer", "architect", "manager"],  
};  
// Use the stringify() method on the object  
let queryString = querystring.stringify(obj);  
console.log("Query String 1:", queryString);  
obj = {  
    name: "Parag",  
    access: false,  
    role: ["editor", "HR"],  
};  
// Use the stringify() method on the object with sep as `, ` and eq as `:` 
queryString = querystring.stringify(obj, ", ", ":");  
console.log("Query String 2:", queryString);  
// Use the stringify() method on the object with sep as `&&&` and eq as `==`  
queryString = querystring.stringify(obj, "&&&", "==");  
console.log("\nQuery String 3:", queryString);

Now, run the command node querystring.js from the Integrated terminal in VSCode or any terminal. Note that you need to be inside the folder NodeJS, which we had created earlier. The output of the same will be below.

Query String 1: name=nabendu&access=true&role=developer&role=architect&role=manager
Query String 2: name:Parag, access:false, role:editor, role:HR        
Query String 3: name==Parag&&&access==false&&&role==editor&&&role==HR

querystring.decode() Method

The querystring.decode() method is nothing but an alias for querystring.parse() method. In our parse example, we can use it. So, add the below code in querystring.js file, which we created earlier.

// Import the querystring module 
const querystring = require("querystring");  
// Specify the URL query string to be parsed  
let urlQueryString = "name=nabendu&units=kgs&units=pounds&login=false";  
// Use the parse() method on the string  
let parsedObj = querystring.decode(urlQueryString);
console.log("Parsed Query 1:", parsedObj);

As earlier, run the command node querystring.js from a terminal. And the output will be same as that with querystring.parse() method.

Parsed Query 1: [Object: null prototype] { name: 'nabendu', units: [ 'kgs', 'pounds' ], login: 'false' }

querystring.encode() Method

The querystring.encode() method is nothing but an alias for querystring.stringify() method. In our stringify example, we can use it. So, add the below code in querystring.js file, which we created earlier.

// Import the querystring module  
const querystring = require("querystring");  
// Specify the object that needed to be serialized 
let obj = { 
name: "nabendu", 
access: true,  
role: ["developer", "architect", "manager"],  
};  
// Use the stringify() method on the object  
let queryString = querystring.encode(obj);  
console.log("Query String 1:", queryString);

As earlier run the command node querystring.js from a terminal. And the output will be same as that with querystring.stringify() method.

Query String 1: name=nabendu&access=true&role=developer&role=architect&role=manager

querystring.escape(str) Method

The querystring.escape() method is used by querystring.stringify() method and is generally not used directly.

querystring.unescape(str) Method

The querystring.unescape() method is used by querystring.parse() method and is generally not used directly. 

Summary

In this article we learnt about the useful query string module in Node.js, which is mainly used to parse URL query strings into Object format and also to change an object to URL query strings.

Nabendu

Nabendu Biswas

Author

Nabendu Biswas is a Full Stack JavaScript Developer, who has been working in the IT industry for the past 16 years and has worked for world’s top development firms, and Investment banks. He is a passionate tech blogger. He is also a tech youtuber and loves to teach people JavaScript. He is also an Apress author with three Gatsby books published. 

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The transporter variable is storing the mail object.  let transporter = nodemailer.createTransport({       host: "smtp-relay.sendinblue.com",       port: 587,       auth: {         user: process.env.USER,         pass: process.env.PASS,       },     });Open SMTP tab in your sendinmail account and cross check the host and port.In the auth object, we are passing the user and password from the environment variable.Send the mail using sendMail method.// send mail with defined transport object     let info = await transporter.sendMail({       from: from, // sender address       to: to, // list of receivers       subject: subject, // Subject line       text: text, // plain text body     });It returns a Promise of Send Message Info. In the end, it is returning the message id.  Express Route The sendEmail method is ready.  Let’s create an express route for it. This is the API endpoint using which we can access the sendEmail function. Open the mail-api.js in the code editor and paste the below code. constexpress = require("express");  constrouter = express.Router();  constsendMailMethod = require("../src/send-mail");  // Post request to send an email  router.post("/sendmail", async (req, res) => {  try {  constresult = awaitsendMailMethod(req.body);  // send the response  res.json({  status:true,  payload:result          });      } catch (error) {  console.error(error.message);  res.json({  status:false,  payload:"Something went wrong in Sendmail Route."          })      }  }) module.exports = router; The express framework provides a Router method to create different HTTP methods. Here, we have created a POST method to send the mail.  Instead of extracting the req.body parameters, we passed it as it is. The benefit of this approach is that if there is a change in argument or position, then we would not have to change it in all the places.  In the end, we are exporting the router. Now, this can be used in the server side. Express Server Now, it’s time to create a server and expose the routes on it. Open the index.js and paste the below code in it. "use strict";  constexpress = require("express");  constbodyParser = require("body-parser");  constcors = require("cors");  constapp = express();  constmailAPI = require("./routes/mail-api.js");  // Express body parser  app.use(cors());  app.use(bodyParser.json());  app.use(  bodyParser.urlencoded({  limit:"50mb",  extended:false,  parameterLimit:50000    })  );  // use the routes specified in route folder  app.use("/api/v1", mailAPI);  constport = process.env.PORT || 4444;  //listen to the server  app.listen(port, function () {  console.log(`listening to the port ${port} .....`);  }); The route is available on /api/v1/sendmail. Run the server. $ node index.js  listening to the port 4444 ..... Send an Email using API There are couple of methods to test the API. Using the Postmanor using the Curl command. Using Postman Install Postman on your machine if it is not installed. Create a POST request. URL: http://localhost:4444/api/v1/sendmail Content Type: JSON – Body tab > select JSON from dropdown Request Body:  {  "from": "hello@schadokar.dev",  "to": ["shubham@schadokar.dev"],  "subject": "Mail from Nodemailer",  "text": "Sending an email using nodemailer package."  } Hit Send. Make sure the server is running. On Success, the response returns the message id. Now, check your mailbox. If you are unable to find the mail in your Inbox, check the promotional mail folder. Using Curl CommandOpen the terminal or cmd.In the below curl command, change the parameters and enter.curl -X POST http://localhost:4444/api/v1/sendmail -H "Content-Type:application/json" -d '{"from": "hello@schadokar.dev","to": ["shubham@schadokar.dev"],"subject": "Mail from Nodemailer", "text": "Sending an email using nodemailer package."}'In this example, we have sent a simple text message to the recipient.  Nodemailer also gives you an option to send the message in HTML format. Mail template Open the mail.html in the code editor and paste the below code. 
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Learn Nodemailer Module in Node.js

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Full Stack Development – the Hottest Developer Skill for the Digital Age

With over 1.7 billion websites worldwide and 4.54 billion people using the internet actively, the need for heightened customer experience is on the rise. This is one of the major reasons why professionals who are adept at handling both the client-side and server-side interfaces of an application/website have become more important than ever. It has been estimated that by the next decade, there will be 300,000 new developer jobs in US. The Full Stack developer role is the No.1 position to be filled in 2020 according to 38% of hiring managers. This is closely followed by the role of a back-end developer.Handsome pay packagesThe average annual salary for a full-stack developer is about $110,737 per annum. Even beginners are offered about $58,000 per year and on the other hand, experienced professionals would earn up to $188,253 per year.These professionals are paid handsomely because enterprises are aware that a full stack developer does the job of two professionals (back-end and front-end developer).Plenty of growth opportunitiesAs per reports by The United States Bureau of Labor Statistics, full-stack development job opportunities will increase from 135,000 to 853,000 by the year 2024. This is quite promising for aspiring full stack developers as an ocean of opportunities will be available for them in both startups as well as in multi-national organizations.Skills to become a Full Stack developerBecoming a full-fledged full stack developer is not child’s play. It takes a wide range of skills to become a good full stack developer. Below are the mandatory skills:Front-end skills: They should be well-versed with basic front-end technologies like HTML5, CSS3, and JavaScript. They should also be comfortable working with front-end frameworks or third-party libraries such as JQuery, SASS, and AngularJS.Programming languages: They should be aces in at least one server-side coding language like Java, Python, Ruby or .Net.Databases: They should be efficient at handling data from databases like MySQL, MongoDB, Redis, Oracle and SQLServer.Version control systems (VCS): Full stack developers must be aware of Git so that they can make appropriate changes to the codebase.Basic design skills: Awareness about the basic prototype design and UI/UX design is essential to become a successful full stack developer.Server and API: They should have adequate exposure to Apache or Linux servers as well as web services.The way forward for Full Stack developersThe growing demand for full-stack developers is due to the ample benefits they offer to organizations. With technology evolving at a rapid pace, foresighted companies will keep adding them to their workforces. Full stack development became the No.1 developer skill because these developers are trained to multi-task various technologies and products. For aspiring full stack developers out there, now is the best time to make the most of these opportunities.Real products require real challenges. Check out our live online workshops and build your portfolio of projects.
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Full Stack Development – the Hottest Develop...

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What are React Component Lifecycle Methods

React is the most popular JavaScript library used to create interactive UI for web applications. It is a component-based library where different parts of a webpage can be encapsulated by components which are quite easy to create, manage and update. React allows us to create Single Page Applications which maintain the state of individual components on an app without having to reload it.  What are React Components? Developers who are new to JavaScript libraries and frameworks like React and Angular might ask the question, “What is a component?” Well, in very simple words, a component is a unit of code which includes JavaScript and HTML to build a part of a web page. It acts like a custom HTML element. It is reusable and can be as complex as you want it to be. For example, imagine that you are creating a very basic application with header, footer, and body. The header can be a component; the footer can be another component and the body can be yet another one or even might consist of multiple components.One of the most useful characteristics of React is its ability to integrate reusable components in a project. Reusability is the characteristic of a component which allows it to be used again, thereby reducing the amount of code a developer has to write. In our example here, the header can be a reusable component and can be used on all the pages of the application, which makes it easy to maintain and update. What does a component look like? Here is a simple example of a react component which contains a simple form. This is a class-based component. React also supports function-based components. As you can see in the code below, App is a user-defined class which inherit from React’s Component class and it has a render method which returns HTML code. As the name suggests, the render method returns and renders HTML to our browser. Every component has to return HTML which is rendered to the user’s browser by render method.import React, { Component } from 'react';  class App extends Component {    handleChange(event) {      this.setState({value: event.target.value});    }    render() {      return (                              Username:                                            Password:                                              );    }  }    export default App; In the above example, we have created a login form where there are 2 input boxes for the user to enter their username and password and then submit the form. We have assigned an event handler to form which will handle the login event in component.We have exported our component (using export default App) so that it can be rendered inside other components.This is a very basic example of component, but this can be as complex as you want it to be. But it is always advised to make your component independent and it should represent only a part of your page which can be reusable as well. It can return complex HTML included with JavaScript to handle complex features in your application.Component as a class React allows us to create component in the form of class as well as functions. While creating component as class you need to define a class which extends React.Component class. Component class has many features which the deriving class can use to maintain the state throughout the lifecycle. In case you want to have more custom features, you can create your own base component class which derives from Component class, and then your component classes can derive from your base component class. What do we mean by Component Lifecycle?Lifecycle of a component is the set of different stages (also known as lifecycle hooks) a component goes through while it is active. Stages could be when a component is created or when any changes are made to the component and many others. There are different methods executed by React at different points of time between when a component is created and at the end when it is destroyed and not in use. One such hook or method we have already seen in the code above, which is render(), and it is executed by React to render the component. We can override these methods and perform certain tasks in those methods, but every lifecycle serves a different purpose and it can be a nightmare if we ask them to do something that they aren’t supposed to or are not very good at. As a developer we should be aware of what those different stages are, what happens in those stages, in what order they execute and how we can make the best use of it. Understanding the lifecycle of components also helps us predict behavior of a component at different stages, which makes it easier to work with them. Managing a large set of components in an application can get you in trouble if you do not know how they work behind the scenes.Props and State Before we start with lifecycle hooks, lets understand what props and state are as they are most commonly used properties in component classes. Props It is a keyword which means properties. Props are used by callers of components to pass properties to the called component in a uni-directional flow. For example, if Parent component renders child component, it can define props and pass them to the child component which is then available and accessible by this.props. Another thing to note here is that props is a ready-only attribute which means data which is passed by parent should not be changed by client components. State State is a plan JavaScript object which defines the current state of any component. It is user defined and can be changed by lifecycle hooks. Ideally state should contain only data which is going to be rendered on DOM. State has getter and setter methods this.getState() and this.setState() which as the names suggest are used to access and update State. It is good practice to use setState method to update State and treat State as an immutable JavaScript object.Since there are many lifecycle hooks a component goes through, it would easier to understand if we start with the hooks which are executed when a component is created.Lifecycle hooks while Mounting [These lifecycle hooks are executed in order as listed, when a component is created]constructor(props) This is not a component lifecycle hook, but it is important to mention here and to be aware that Constructor is executed before it is mounted. Constructor receives props(properties of a component) as an argument which then can be passed to base class using super keyword if we define the constructor.  It is not mandatory to define constructor in component class, but if you do to perform any logic, then you need to call base constructor using super keyword.  Mainly constructors are used: To Setup local state of component with this.state To bind event handler methods. This is what a simple constructor would look like.import React, { Component } from 'react';  class App extends Component {    constructor(props) {      super(props);      this.state = { value: 0 };      this.handleClick = this.handleClick.bind(this);    }  } this.state should be called only inside constructor, to update the state in other methods use this.setState() method.  If constructor is required to do any heavy tasks, it will impact the performance of component, and you should be aware of this fact.  getDerivedStateFromProps(props, state) After constructor, this lifecycle hook is called before render method is executed. It is called while mounting as well as whenever props have changed. This is not very commonly used, only in cases where props can change, and you need to update state of the component. This is the only use case where you should implement this lifecycle hook.This method is executed on every render and cannot access component instance.import React, { Component } from 'react';  class App extends Component {    getDerivedStateFromProps(props, state) {      if (props.value !== state.prevValue) {        return {          prevValue: props.value        };      }      return null;    }    }render() This is the method which is required to be implemented in component class. It can access props and state. This is where you can write your html and jsx code. You can also render child components in this method which will then be rendered as well. Before completing the lifecycle of parent, lifecycle of all child components will be finished. All this html and jsx is then converted to pure html and outputs in DOM. JSX is a JavaScript extension which creates React elements. It looks more like template language but it is empowered by JavaScript which allows it to do a lot more. It can embed expressions . JSX has different set of attributes than what we have in html. For example, while creating html using JSX you need to use attribute “className” instead of class. This is what a typical render method looks like:import React, { Component } from 'react';   class App extends Component {   render() {         return (        Click to go Home { this.state.home }       Go to Home         );   } } Alternatively you can also use React.createElement() method to create html using JSX.const element = React.createElement(       'h1',       {className: 'hello'},       'Hello, world!'     );componentDidMount() As the name suggests, componentDidMount() is invoked after the component is mounted, which means html has been added to DOM tree. It is a very commonly used lifecycle hook, as it allows you to do a lot of things including causing side-effects, setting up any subscriptions, or loading data from external endpoints. If you setup any subscription using this method, make sure to unsubscribe them in componentWillUnmount() lifecycle hook. You shouldn’t update state in this method using this.State() as it may cause performance issues. For assigning initial state you should use constructor(). import React, { Component } from 'react';  class App extends Component {    componentDidMount(){    // Component is rendered and now external calls can be made.      this.getDataAfterTimeOut();    }    getDataAfterTimeOut(){      setTimeout(() => {        this.setState({          data: 'Data is fetched'        })      }, 1000)    }  } Lifecycle hooks while Updating [Next set of lifecycle hooks are executed while a component is updating which can be caused by changes to props(properties) or state of component. These are invoked in order as listed below.] getDerivedStateFromProps(props, state) We have already talked about this. This is invoked every time a component is changed or updated. Any changes in properties or state which causes the component to be changed will invoke this method. shouldComponentUpdate(nextProps, nextState) shouldComponentUpdate() is invoked before rendering (not on initial rendering) but only when props or state has been changed. Even though it is not recommended you can use this lifecycle hook to control the re-rendering. This can lead to performance issues as well as bugs, so be careful while doing that.  In this method nextProps can be compared with this.props and nextState can be compared with this.state. This method can return true or false depending on whether you want to continue rendering by skipping the next lifecycle hooks. In either case it can’t prevent re-rendering of child components. Note that this method defaults to true which will not skip rendering and next lifecycle hooks and continue with execution. import React, { Component } from 'react';  class App extends Component {    shouldComponentUpdate(nextProps, nextState) {  // This value will determine if lifecycle execution is to be skipped or continued.      return nextProps.value != this.props.value;    }  } render() After shouldComponentUpdate lifecycle hook render is called, which we have already talked about, it prepares html and jsx code which then outputs to DOM. getSnapshotBeforeUpdate() getSnapshotBeforeUpdate() is invoked right before the recent changes are added to DOM. This lifecycle hook gives us an opportunity to capture any details we need from the DOM before it is updated with new content. For example, if you want to know the scrolling position of the user, which should be restored after the DOM has changed. Use cases for this lifecycle, while rare, can be of great value at times. The snapshot value which is captured and returned by this hook is passed as a parameter to another lifecycle hook componentDidUpdate() which we will talk about next. import React, { Component } from 'react';  class App extends Component {    getSnapshotBeforeUpdate (prevProps, prevState) {  // implementing this method here allows us to capture the snapshot of current dom tree.      if (this.state.value != prevState.value) {        return table.scrollHeight - table.scrollTop      }      return null    }  }componentDIdUpdate(prevProps, prevState, snapshot) componentDidUpdate is invoked when DOM is updated. It is only called on update, not on initial rendering. You can use this method to make data requests after checking if props have changed. You can also call setSatate() in this method, but make sure to wrap that in a condition else it will cause an infinite loop forcing re-rendering and affecting performance issues. Also it should be noted that value for snapshot will only be available if you have implemented getSnapshotBeforeUpdate() in your component; else value for snapshot will be undefined. Here is an example of componentDidUpdate. This is a very basic example where we have captured snapshot by implementing get Snapshot Before Update lifecycle hook. After that componentDidUpdate is invoked and content is overwritten with new dataimport React, { Component } from 'react';  class App extends Component {    getSnapshotBeforeUpdate(prevProps, prevState) {  // implementing this method here allows us to capture the snapshot of current dom tree.      document.getElementById("divContent").innerHTML =      "Before the update content is " + prevState.content;    }    componentDidUpdate(prevProps, prevState, snapshot) {  // You can access snapshot here to get data from dom before it was updated.      document.getElementById("divContent").innerHTML =      "New content updated " + this.state.content;    }  } import React, { Component } from 'react';  class App extends Component {    getSnapshotBeforeUpdate(prevProps, prevState) {  // implementing this method here allows us to capture the snapshot of current dom tree.      document.getElementById("divContent").innerHTML =      "Before the update content is " + prevState.content;    }    componentDidUpdate(prevProps, prevState, snapshot) {  // You can access snapshot here to get data from dom before it was updated.      document.getElementById("divContent").innerHTML =      "New content updated " + this.state.content;    }  } UnMounting [This is where lifecycle of a component ends when component is destroyed and removed from DOM. While Unmounting React gives us an opportunity to do something before component is destroyed, it can include clearing objects which have occupied memory to avoid memory leaks.] componentWillUnMount() componentWIllUnMount() is executed right after component is unmounted which means it is removed from DOM and destroyed. But before it is removed and destroyed, React gives us an opportunity to perform any cleanup we want to. For example, you might have setup subscriptions initially in componentDidMount() which you should unsubscribe when component is destroyed to avoid memory leaks in your application. You can also remove event listeners which were subscribed before. In this lifecycle hooks you should not update state of your component because component is not going to re-render now.import React, { Component } from 'react';  class App extends Component {    componentWillUnmount() {  // Component will be removed from DOM now.        // Unscubscribe subscriptions and events here.  document.removeEventListener("click", this.handleSubmit);    }  }Conclusion In this article we talked about React, its components and its different lifecycles. It is very crucial to understand the different opportunities that React provides through these lifecycle methods. There are many rules we need to follow while working with these hooks. Making them do something they can’t handle can cause performance issues or even infinite loops at times.  These lifecycle hooks work with props and state which are the most used properties of component class. Changes in state and props trigger different lifecycle hooks and even re-render the dom which is something you should be aware of. These lifecycle hooks are provided to intercept the different stages a component goes through and make the best use of it, but without understanding how they work it can break your application by causing performance issues or memory leaks. Hope this has been helpful. 
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What are React Component Lifecycle Methods

React is the most popular JavaScript library us... Read More