Trying to use the const keyword to declare a variable in the imperative for loop will result in a TypeError: The reason is that the declaration is only evaluated once before the loop body starts. Your function will never get overridden by some other js file having same function name in case you declare it as const. JavaScript const variables must be assigned a value when they are declared: Meaning: An array declared with const must be initialized when it is declared. Variable illustration <A> TYPES OF VARIABLE. In JavaScript, a function is a value, so we can deal with it as a value. superagent. Then you have to write the function logic between curly brackets { } Here is an example of how to write a . So we can work with it like with other kinds of values. What is the most efficient way to deep clone an object in JavaScript? It is a bit complex, and it would require more explanation to fully understand than I want to include here. It appears to be used to create immutable . Given below are the examples of Javascript constants: Assigning or changing the value of constant EMPLOYEE_NAME in the above code. So lovely. Your email address will not be published. abcjs je open source knihovna hudebnch notac zaloen na JavaScriptu iroce pouvan k vykreslovn notace ABC. Can be used to create a self-invoking function (Immediately Invoking Function Expression), executed as soon as the function is defined. Let us take an example. In JavaScript, a constructor function is used to create objects. Syntax: number.toFixed (x) Parameter: x is the desired number of decimal places. JavaScript const: Declaring Constants in ES6, Understanding Pass-By-Value in JavaScript, Immediately Invoked Function Expression (IIFE), Removing Items from a Select Element Conditionally. However, the block-scoped variables declared by the const keyword cant be reassigned. The variables declared by the const keyword are blocked-scope and cannot be redeclared. Operators 7. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. When I first saw this I was like: Even though Babel is free. The first argument is a regular expression/pattern or string that defines what needs to be replaced. Learn more about Teams So you might doubt why we should capitalize when we have the keyword const or do we need to capitalize on anything that is declared using the const keyword. Teams. What's more, before using a function, you must define it in the scope where you wish to invoke it. Like the let keyword, the const keyword declares blocked-scope variables. However, its functional scope was a problem. What Is a Function Declaration in JavaScript Function Declaration means defining a function with a name and parameters. Connect and share knowledge within a single location that is structured and easy to search. Then I convert it to typescript. Nonetheless, a procedure must take an input and return an output to qualify as a function. Use `const` and make your JavaScript code better | by Alberto Gimeno | DailyJS | Medium Write Sign up Sign In 500 Apologies, but something went wrong on our end. Object properties can be changed but the reference to an object cannot be changed. 1. var (Can re-declared & re-initialize) 2. const (Can't re-declared & re-initialize, can update array values by using push) 3. let (Can re-initialize but can't re-declare) This site uses Akismet to reduce spam. If you do want to globally scope width, we need to declare it outside the function like so, and update it inside the function: var width; function setWidth() { width = 100; console.log(width); } setWidth(); console.log(width); Generally, it's probably not what you want to do. We have two new JavaScript keywords, i.e., let and const. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Getting Started with JavaScript 3. For nearly twenty years, variables were declared with var (not counting parameters and function declarations, of course). The function will never get overridden by some other js file having same function name in case you declare it as const. log (); // } console. Constant EMPLOYEE_NAME has already been declared, which is a Syntax error. We can use the arrow function expression and assign it to a variable. In JavaScript, a function resembles a procedure, i.e., a set of statements that calculates a value or performs a specified task. Today, we shall look into JavaScript Constant. . In the above code, the function sum will be available before executing the first line of the code. And then? When would I give a checkpoint to my D&D party that they can return to if they die? You can define the JavaScript function with the const keyword. JavaScript is a very flexible language. How do I return the response from an asynchronous call? Teams. We have seen what JavaScript Constant and its syntax is. But it's still a value. All Rights Reserved. However, we cannot reassign the array colors to another array. Example 1: It describes that the const variable cannot be reassigned. By the end of this section, you will be able to: Derive the kinematic equations for constant acceleration using integral calculus. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked, Effect of coal and natural gas burning on particulate matter pollution. Overall, constants can be declared whether using an upper case or lower case, the most used and common convention is to all upper case letters. JavaScript let: Declaring Block-Scoped Variables. through a variable declaration ). You may also have a look at the following articles to learn more . It adds functionality to a webpage, and javascript is utilized by around 97 percent of all websites on the internet. Fixed @t.niese. Constant Necklace Male. The function accesses the outer world using its parameters. Lets create a function with the name sum, which will take two arguments. Let's talk about composing functions. C# Programming, Conditional Constructs, Loops, Arrays, OOPS Concept, This website or its third-party tools use cookies, which are necessary to its functioning and required to achieve the purposes illustrated in the cookie policy. Learn more about Teams JavaScript being an interpreted language, previously we used to store values in variables even though they are constant value. Creator Notes. A variable declared inside a function using these keywords is not accessible outside the function. When we compose functions together, the main goal is to take a function and combine it with another function--so that when both of them are . Using const without initializing the array is a syntax error: Example This will not work: const cars; cars = ["Saab", "Volvo", "BMW"]; Arrays declared with var can be initialized at any time. TypeError: Assignment to constant variable. Commented lines, you people can just try out by uncommenting it. Different Types of console.log() 5. const and let are recent additions to JavaScript. Creating Local Server From Public Address Professional Gaming Can Build Career CSS Properties You Should Know The Psychology Price How Design for Printing Key Expect Future. One of the interesting parts of using constants is to know about the Object.freeze() method. 217231/const-in-javascript-when-to-use-it-and-is-it-necessary An example was function sayThanks (name) { console.log ('Hey thank you ' + name + ' for buying our shit!'); later we used an argument to change the name in . Streams3, a user-land copy of the stream library from Node.js. Refresh the page, check Medium. Constants can be either global or local to function where it's being declared. At times, we can write the arrow function like this: const MyComponent = () => (.) Use const when you declare: A new Array A new Object A new Function A new RegExp One main thing is capitalization, which we surely help to know which are the constant values and the variables in the program. If you attempt to reassign a variable declared by the const keyword, youll get a TypeError like this: Unlike the let keyword, you need to initialize the value to the variable declared by the const keyword. Due to this, we can call the function before the declaration. That's the applied functional scope. Q&A for work. 3) "is it considered bad practice for any reason" I guess it hasn't been around for long enough to say anything about that but I don't see why this should be pad practice. Create only read only reference to value. This article will discuss the different use of constant function expressions in JavaScript. Summary: in this tutorial, youll learn how to define constants by using the JavaScript const keyword. Javascript const vs. function I have been performance-optimizing our app recently and as such, I have been getting into the nitty-gritty of Javascript. monthtomonth rent increase notice roblox bypass audio bot inhome pet euthanasia brooklyn best lobster roll belfast maine concord blue devils bingo schedule . We have two new JavaScript keywords, i.e., let and const. Folder structure By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. A property of the const variable is that we cannot change or modify its value throughout the program. Hence to declare a constant variable, need to use the keyword const instead of let.. Published Jun 06 2019, Last Updated Aug 27 2019. JavaScript const variables must be assigned a value when they are declared: Correct const PI = 3.14159265359; Incorrect const PI; PI = 3.14159265359; When to use JavaScript const? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. The main idea of functional programming revolves around functions. In other words, you cant reassign them to different values. For a const in JavaScript, an initializer is required. 2022 - EDUCBA. For reference, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions Objects In JavaScript 11. For example: Even though the person variable is a constant, you can change the value of its property. 1. The const keyword creates a read-only reference to a value. Async functions may also be defined as expressions. Cannot be Hoisted. Surely, a function is a special value, in the sense that we can call it like sayHi (). The only differences are: The keyword const was introduced in JavaScript ES6. It means that you can change their values anytime you want as shown in the following example: However, variables created by the const keyword are immutable. Functions In JavaScript 10. The value of a constant can't be changed through reassignment (i.e. With this, we conclude the topic JavaScript constant. The definition will remain exactly the same, whatever be the case. function test() {} However with const and let functions are scoped otherwise they are global or semi scoped when using var. const CONSTANT_NAME = value; Code language: JavaScript (javascript) By convention, the constant identifiers are in uppercase. .json () const fetched = await fetch ("/url", { method: 'GET',});const fetchedJson: object = await fetched.json ();console.log (fetchedJson) . Websites use JavaScript and third-party libraries on the client-side. We must know that the JavaScript constant function creates a scope that can be global or local depending on the block on which it is declared. Comments (2) Description. For this reason, if we pass the function using ''this'' as a callback, that function will not look for global scope but will have the value of ''this'' bound to it. Ready to optimize your JavaScript with Rust? SCHOOL Object is const and frozen; no other details will be mutable, but the SCHOOL.studentAddress Object can be mutable, new properties can be added. The code above shows its string representation, which is the source code. Function overloading with implicit conversion on const reference and pass by value in C++ . Copyright 2022 by JavaScript Tutorial Website. Your function should handle two arguments, being express.Request and express.Response instances respectively. are explain below: Updating the Display Screen Initially, the calculator screen will show nothing. The second argument can either be a string that is the replacement, or a function that will be invoked to create the replacement. What does "use strict" do in JavaScript, and what is the reasoning behind it? New Method Object.freeze(), which will help you to add properties to an object for which we have seen an example illustrating the same. : +49 (0) 9673 255 Fax: +49 (0) 9673 475 pertl_reisen@t-online.de var functionName = function() {} vs function functionName() {}. My previous attempts used to break leaflet because the async array would output with undefined in the console, but i've fixed it now which means it's another issue. I write one javascript function. We can call a function with any number of arguments of any type and at any time. const func = function () { }; It makes the function immutable, so you don't have to worry about that function being changed by some other piece of code. Function Declaration means defining a function with a name and parameters. What is the difference between "let" and "var"? var is used instead of const or vice versa? (not not) operator in JavaScript? Do comment if you have any doubts or suggestions on this JS function topic. const helloWorld = () => 'Hello World!'; This is a function expression in ES2015 JavaScript and it's sexy as hell. When I convert I am getting 3 errors. Second, using an arrow function doesn't have it's own lexical context, so it won't have a scoped this and can't be used as a constructor. (for, if, function ) function {var = 'Kim'; console. You can use fat arrow syntax, which is shorter & cleaner. Re-declaring of a const variable inside different block scope is allowed. Variables (var, let, const) and Its Scope 6. let, const {}. However, Polyline is not outputting anything, and it works when I enter a simple test constant like: const test = [ [1, 2], [3, 1], ]; How can I approach this problem? chalk. THE CERTIFICATION NAMES ARE THE TRADEMARKS OF THEIR RESPECTIVE OWNERS. JavaScript has a concept called Hoisting which allows for variables and functions to be declared on lines below the lines where they are used. So, instead I will show you what it means in terms of our components. Learn how your comment data is processed. What is the difference between using const and function in javascript? One thing to remember is that, unlike the var variables, JavaScript constants do not become a property for the window object. ES6 provides a new construct called forof that allows you to create a loop iterating over iterable objects such as arrays, maps, and sets. Here, the value of 'this.name' is rightly picked as 'Rohit'. History of JavaScript 2. The values inside the const array can be change, it can add new items to const arrays but it cannot reference to a new array. If you want any of the objects to be immutable, we need to use Object.freeze() method, so by freezing the object properties of the object are frozen but not the Object references by the properties. Can virent/viret mean "green" in an adjectival sense? For example, the companyobject is constant and frozen. In other words, a new score constant is created in each iteration. Control Flow (If else ,loops, break, switch) 9. const sentence = 'The world is a cruel. Which both prevents reassignment and creates a lexical context. Variables that are defined with const behave as let variables cannot be reassigned or redeclared. Here is the code I am using to make the request const requestOptions . Degree in Computer Science and Engineer: App Developer and has multiple Programming languages experience. Using arrow functions takes care of this binding for you. What Is a Function Declaration in JavaScript, What Is a Function Expression in JavaScript, Difference Between JavaScript Inline and Predefined Functions, Export All Functions From a File in JavaScript. Popular in JavaScript. logMessage . // Function definition of a non-constant variable can be changed, // when we try to chanage const value we will get error. It is true irrespective of whether you use var, let, or const. The definition will remain exactly the same, whatever be the case. How to connect 2 VMware instance running on same Linux host machine via emulated ethernet cable (accessible via mac address)? How to build a javascript calculator? Featured Artist. Code: 2 1 First, const prevents reassignment of the name square while function does not. I want to be able to quit Finder but can't edit Finder's Info.plist after disabling SIP. Q&A for work. If you see the "cross", you're on the right track. Let's see what one-time assignment means: // We're declaring PI to be a constant variable. You can than obtain its value or compare its value letter: if (userInput === password) { console.log ('That is correct'); } On the other hand function is used to create a section of code that can be reused many times: function add (n1, n2) { let result = n1 + n2; console.log (result) return result; } Required fields are marked *. readable-stream. Books that explain fundamental chess concepts. The async and await keywords enable asynchronous, promise-based behavior to be written in a cleaner style, avoiding the need to explicitly configure promise chains. To define a function, you must use the function keyword, followed by a name, followed by parentheses ( ). Redeclaring the constant and reassigning the value. Notify me of follow-up comments by email. JavaScript value that cannot be altered by programming during execution is a Constant, simple to work on, right? What is the !! Should teachers encourage good students to help weaker ones? Does integrating PDOS give total charge of a system? The following example causes a SyntaxError due to missing the initializer in the const variable declaration: The const keyword ensures that the variable it creates is read-only. Are 'Arrow Functions' and 'Functions' equivalent / interchangeable? Basics of JavaScript 1. Second, using an arrow function doesn't have it's own lexical context, so it won't have a scoped this and can't be used as a constructor. So it's a matter of needs on a given point on your software. I can't do this fix. elegant & feature rich browser / node HTTP with a fluent API. So terse. Here we discuss What JavaScript Constant and its syntax are, along with a few simple examples. Main point is that how to decide which one identifier should be used during development. It is simply a way to store a value. Connect and share knowledge within a single location that is structured and easy to search. However, if a constant is an object or array its properties or items can be updated or removed. We can pass it as an argument, return from a function, store it in a variable. If in case the keyword const is removed, the identifier is represented as a variable. You can use fat arrow syntax, which is shorter & cleaner. This is because the function is now not looking at the global scope for the value of . It will make the code shorter and more understandable. Always declare a variable with const when you know that the value should not be changed. This article will discuss the different use of constant function expressions in JavaScript. const obj = { msg: 'Hi', logMessage: function () {} } logMessage logMessage . Like the let keyword, the const keyword declares blocked-scope variables. The const variable creates only reference to the value. Yes, but we have something called Naming Convention for these Constants. Some of the constant references can change depending upon the programming logic. In JavaScript, it can look like this: const f = console.log const g = (str) => `Hello, $ {str}` const sayWord = (x) => f(g(x)) sayWord('bryan') // "Hello, bryan". Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. However, all the . Variables that are defined with const behave as let variables cannot be reassigned or redeclared. JavaScript Function Parameters Learn the basics of JavaScript Function Parameters. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. First, const prevents reassignment of the name square while function does not. in javascript, variables that are created with the const var or const in javascript declare const in javascript js define function as const declare function const javascript code const js where to declare constant variable js js constant variable declare what does const { } = do in js const in a function javascript what is const in jas js A tag already exists with the provided branch name. ALL RIGHTS RESERVED. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? Name of a play about the morality of prostitution (kind of). Where does the idea of selling dragon parts come from? However, you cannot reassign a different value to the person constant like this: If you want the value of theperson object to be immutable, you have to freeze it by using the Object.freeze() method: Note that Object.freeze() is shallow, meaning that it can freeze the properties of the object, not the objects referenced by the properties. If we put normal parenthesis after the arrow we don't need to write the return. Central limit theorem replacing radical n with n. What's the \synctex primitive? For reference, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions. What is the JavaScript version of sleep()? . Then, we can change the arrays elements by adding the green color. function PostgresDBAdapter() { var db = pgp . You want to keep your variables inside of your function. However, your addArticle function doesn't conform to express route handler signature. - Function Declarations Function Expression is very similar to function declaration. In JavaScript, const does not mean constant, but one-time assignment. A few Functions and Argument vs Declaring Individual Variables (Const/Let) So in today's Javascript Syntax lesson, I learned about using Functions and using Arguments with in functions. This is a guide to Javascript Constants. Installation Help Report this Download. JavaScript const function comes with some great advantages that make it superior. Published Apr 29, 2022. But the company.address object is not immutable, you can add a new property to the company.address object as follows: In this example, we declare an array colors that has one element using the const keyword. To differentiate among variables and constants, ECMAScript 6 follows some guidelines for naming constants. Illustrated a few simple examples which you people can try your hands on. The variables declared by the let keyword are mutable. Join the waiting list to reserve your spot in my 10-weeks cohort course and learn the fundamentals, HTML, CSS, JS, Tailwind, React, Next.js and much much . You seem to ask multiple questions: 1) "I am interested if there are any limits to what types of values can be set using const in JavaScript" No. async connectionLazyInit(port) { const db = pgp ({ host: . The const keyword creates a read-only reference to a value. Copyright 2014EyeHunts.com. Always use let and const instead. What is the difference between Bower and npm? Editor vyuv pro zpracovn vstupn notace abcjs (ABC JavaScript). What is the difference between call and apply? Whenever you execute a function in JavaScript, it creates a new execution context comprising execution thread and a local memory where anything defined in a function is stored. Add a new light switch in line with another switch? Not the answer you're looking for? // Global Variables. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Home JavaScript Tutorial JavaScript const: Declaring Constants in ES6. If you dont intend to modify the score variable inside the loop, you can use the const keyword instead: In this example, the forof creates a new binding for the const keyword in each loop iteration. Assigning a function expression to a const variable ensures that the function definition is unchanged because you cannot change the const variable. All Right Reserved. Because, like the syntax of this method, the variable used with the toFixed method must be a number, you are passing it a non-numeric value. Hence the Missing Initializer Syntax error. Where is it documented? Try it Syntax JavaScript const function comes with some great advantages that make it superior. Assign the function to a variable and execute the function by using the variable name. What is the difference between null and undefined in JavaScript? 2) "Is this valid?" Yes. We haven't looked at it yet, but JavaScript provides a for loop for your iterating pleasure and convenience. Function expressions are not hoisted until the function is declared, so we cannot call the function until the function expression is evaluated. Find the functional form of velocity versus time given the acceleration function. Enthusiasm for technology & like learning technical. Sadly JS is a bit messed up due to some bad decisions and maintaining comparability so it's a bit hard to summarise all differences. HTML . 2 There are several. However, the block-scoped variables declared by the const keyword can't be reassigned. It makes the function immutable, so you dont have to worry about that function being changed by some other piece of code. Naming a constant in JavaScript has some rules for naming a variable, keeping intact the const keyword, and global constants. JavaScript Constants being blocked scoped similar to variables being declared using the let keyword. In JavaScript a function is composed and influenced by many components: JavaScript code that forms the function body The list of parameters The variables accessible from the lexical scope The returned value The context this when the function is invoked Named or an anonymous function The variable that holds the function object If in case you try to change the value of employeeName to Anusha, you will face an error as you cant reassign a constant, Only when the programmer is sure that the variable will not face any changes, then it can be declared using const to guarantee the fact that the variable is a constant. How to Use JavaScript Variables in Functional Scope. The first argument passed to every function is a context object, which is used for receiving and sending binding data, logging, and communicating with the runtime. by using the assignment operator ), and it can't be redeclared (i.e. You can see the error on the screenshot as Assignment to the constant variable online 3.. However, it doesnt mean that the actual value to which the const variable reference is immutable. Const keyword denoting that the variable is a constant which cannot be reassigned or redeclared. JavaScript functions are reusable blocks of code that perform a specific task, taking some form of input and returning an output. Note you can also do: const logWhatever = function( something ) { console.log(something); } Which both prevents reassignment and creates a lexical context. But to summarise all points use function name() {.} 2023 WEB DEVELOPMENT BOOTCAMP starting in days! It's beautiful to look at. Note:TheAll JS Examples codesaretested on the Firefox browser and the Chrome browser. christopher067. Constants can be either global or local to function where its being declared. By convention, the constant identifiers are in uppercase. Your email address will not be published. var function. It's all one line. To replace a string in JavaScript , you can use the replaceAll function . The declared functions are hoisted to the global scope or top of the enclosing block. Create a function expression to calculate the sum of two values and assign it to a non-constant variable, Assigned a new function expression (which will calculate the product of the two values) to the variable, Then, we created another function expression and assigned it to a. Constants are block-scoped, much like variables declared using the let keyword. It's acceptable to pass your route handler however you see fit. // TypeError:Assignment to constant variable. The JavaScript Tutorial website helps you learn JavaScript programming from scratch quickly and effectively. log (); // var function function . const PI = 3.141592653589793; // Any attempt to assign a new value to PI // fails because PI is a constant variable. What is a Const variable in JavaScript A const variable is a variable that has a fixed value and remains the same throughout the program. By closing this banner, scrolling this page, clicking a link or continuing to browse otherwise, you agree to our Privacy Policy, Explore 1000+ varieties of Mock tests View more, Special Offer - JavaScript Training Program (39 Courses, 23 Projects, 4 Quizzes) Learn More, 600+ Online Courses | 50+ projects | 3000+ Hours | Verifiable Certificates | Lifetime Access, JavaScript Training Program (39 Courses, 24 Projects, 4 Quizzes), Angular JS Training Program (9 Courses, 7 Projects), JavaScript Training Program (39 Courses, 23 Projects, 4 Quizzes), Object Oriented Programming in JavaScript, Software Development Course - All in One Bundle. The Function statement declares a function that will only execute when called. Arrow functions create a new scope, otherwise the, Sorry, meant lexical context, not scope. Find centralized, trusted content and collaborate around the technologies you use most. Data Types 4. const countryData = [ { id: 1, . Find the functional form of position versus time given the velocity function. One of the things I thought about is. Please anyone help me. Sims 4 / Accessories / Male / Neklaces. A JavaScript (Node.js) function is an exported function that executes when triggered ( triggers are configured in function.json ). In JavaScript, functions are first-class citizen i.e values. developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions. I'm trying to send a POST request to a firebase cloud function from Vue.js 3 but I can't seem to get the data to actually send. Start Your Free Software Development Course, Web development, programming languages, Software testing & others. rev2022.12.9.43105. What is the difference between these two functions? Are defenders behind an arrow slit attackable? Podvejme se na Funkce abcjs Quick Editor: Okamit pevdjte notaci ve formtu ABC na noty v relnm ase. Hammer 28 D-93464 Tiefenbach Tel. A const arrow function needs to be declared before calling it, otherwise it's undefined A function can be declared after calling it. Variables that are declared outside the block or function accessed throughout the . We are just declaring the constant EMPLOYEE_AGE and not initializing it with a value. We need to pull the value of dispValue attribute on the calculator screen which should be '0' by default. An async function is a function declared with the async keyword, and the await keyword is permitted within it. JavaScript is a simple programming language that allows website owners to add additional features and functionality. It's a subtle yet important distinction. In JavaScript, const allows you to only modify the value of the array, but the reference to the array cannot be changed. In java-script here are three identifiers. Simple function example code with const keyword and parameters. Const creates a constant which has a read-only reference; there is no meaning that the value the variable is holding is immutable. const MyComponent = () => { return( .. ) } vs. function MyComponent() { return( .. ) } In this form the function syntax is slightly shorter. javascript function parameter A function is a cohesive piece of code coupled to perform a specific task. See the code and the output below. JavaScript . The most common and standard way of returning an object from an arrow function would be to use the longform syntax: const createMilkshake = (name) => { return { name, price: 499 }; }; const raspberry = createMilkshake('Raspberry'); // 'Raspberry' console.log(raspberry.name); This pattern is great, as it allows us to easily add some local. Do non-Segwit nodes reject Segwit transactions with invalid signature? Type Conversions 8. function c++ pass-by-reference overloading It uses an arrow function which is one of the most popular features of ES2015. ES6 has helped people in declaring the const, which is a constant, and added new keywords const and let.. Notice that the const will not work in an imperative for loop. These JavaScript Constants are read-only and declared with const keyword, which gets assigned at the time of declaration. For example, // constructor function function Person () { this.name = 'John', this.age = 23 } // create an object const person = new Person (); Run Code In the above example, function Person () is an object constructor function. To start with any programming logic, we need to first have some constants and variables in hand, which would be useful for the logic. In JavaScript, I recently came across the const keyword.