WebExplanation: In the var tab, when you run the code, you will see there is no error and that we can declare var variables after defining them. }; It can be demonstrated easily with the help of following code snippet: Save the file with name letRedeclare.html and open it in any browser (Chrome, Firefox, or IE). Do you have trouble understanding variable declaration in JavaScript? They started working on adding support for a scripting language, which they named JavaScript to ride the coattails of the hot programming language of the time: Java. In the above example, we take a two-step approach, first declare a variable and then Initialize it, but we can do declaration and initialization at the same time like this -. Subscribe to Developer Insider for top news, trends & analysis, Difference Between var, let, and const in JavaScript, Common Causes of JavaScript Errors and How to Avoid Them, Best Collaboration Tools for Database Developers, Advantages and Disadvantages of OOP in Java, How to Interact With Ethereum Contracts Using JavaScript. Its unusual history makes it one of the most popular languages today, but also one of the most difficult to master. Its syntax looks like below: let is block scoped. However, there were some issues associated with variables declared using the var keyword so it was necessary to evolve new ways for variable declaration. Again, this is because of the block scoping nature of const declarations. It allows you to be immutable with variables. let a = 123; JavaScript Variables JavaScript Variables are among the most fundamental concept in JavaScript, as well as other programming languages. The accepted answer is missing a point: { // document.write("The value of block local variable outside block is: ", localBlockVariable, "
"); // document.write("The value of block local variable inside function is: ", localBlockVariable, "
"); // document.write("The value of block local variable outside function is: ", localBlockVariable, "
"); // The following statement will raise an error, // let globalVariable = 15; // Re-declare in same scope, // This following statement will give error, as const can't be assigned a new value, "The value of block local variable inside block is: ", // This will be considered a new variable, // const globalVariable = 15; // Re-declare in same scope. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. Here are some notes that I took that helped me on this subject. Property of TechnologyAdvice. In the above example, we take the var variable as an example, but we can do Declaration and initialization, same with the let and const. Also comparing const and let to var. 2013-2023 Stack Abuse. The let variable has block scope, meaning let variable will be accessible only inside the block it's declared if we try of access outside of the scope, it will show a Reference Error. var and let are both used for variable declaration in javascript but Variables declared with the "const" keyword maintain constant values and can't change their values during the scope. The var, let, and const is the keyword to declare variables in javascript. ToolsQA.com | All rights reserved, "The value of global variable outside block is: ", "The value of global variable inside block is: ", "The value of block local variable outside block is: ", "The value of global variable inside function is: ", "The value of block local variable inside function is: ", "The value of global variable outside function is: ", "The value of block local variable outside function is: ", // This following statement will give error, as the local function variable can't be accessed outside. The Difference Between var, let and const in JavaScript and Best Practices Joel Olawanle Introduction Programming has always included the definition of data, manipulation of data, and finally displaying data. The Solution. Reserve the use of var for legacy code or specific situations where its behavior is necessary. The difference between let and var is in the scope of the variables they create: Variables declared by let are only available inside the block where theyre defined. Whereas, Without any prior knowledge of JavaScript, doing something as simple as declaring variables can be confusing. For instance: The name that points to "John Doe" is global, and the name that points to "Daniel Joan" is defined within a block. In JavaScript, there are three main ways to declare variables: var, let, and const. They wouldnt be very good variables if they couldnt vary. Prior to the release of ES2015 (ES6), JavaScript variables were only declared using the var keyword; however, with the introduction of ES6, new ways to declare variables, let and const, were introduced. What's the point of declaring a const in JavaScript, Javascript ES6 export const vs export let. Also, open the Chrome dev-tools console to visualize the error raised. There are some main differences that make let a little less permissive, and therefore safer, than var. Here is an example of let variable declaration -. These are a few ways to declare variables. If we would have declared it with the var keyword, then it would have been available, because var has global scope within a function. The const declaration was introduced with ES6, alongside let, and it is very similar to let. And the scope of that variable is only inside the for loop. const are block scoped. It standardized various implementations of JavaScript. They are function scoped and hoisted.Show more. Ltd. , Free Javascript Course - Mastering the Fundamentals, Your feedback is important to help us improve. Implementation 3.1. Now when we try to print the myName variable to the console, it prints successfully, but when we try to print the variable outside the if block, we get a Reference Error. Copyright 2022 InterviewBit Technologies Pvt. (LogOut/ In the example above, we have a variable myAge and an if conditional block, our if condition is true here, so we came inside the if block. When we try to print the myName variable to the console, it shows SyntaxError, which means we can't re-declare the const variable. A variable declared as const maintains a constant value in JavaScript. When we declare a var variable outside the function, it will have a global scope and will be available to use everywhere inside the whole program. Var is an old way of declaring variables. What is the difference between var, let and const? @Jonny Im sure it is. On the other hand, using ES6 const is much like using the let, but once a value is assigned, it cannot be changed. Var. I liked to tackle his questions first thing in the morning because he lived on the East Coast, and I wanted to make sure I wasnt blocking him from getting his work done. Take a look at this example: Youll notice that we declare and assign the foo constant twice, but the execution of our code will not throw an exception. They are all hoisted to the top of their scope. Probably the best way to make an object (shallowly) immutable at the moment is to use Object.freeze() on it. 30 May 2023 15:14:48 Change). The behaviour in is either JS Bin being misleading as to what type of JavaScript is being run, or its a browser bug. 2022 TechnologyAdvice. const should be used when you know that the variables value will not change. Here's the answer var - pre ES6 way to declare variables. forof loop. If you remember declaring a constant variable was a problem in var variable, but const variable solves this problem. Having two main browsers with two separate scripting languages meant that web pages would often only fully function in one browser or the other. It gets hoisted to the top of its scope and initialized undefined. Microsoft entered the scene and introduced the Internet Explorer web browser in 1995 and effectively started the browser wars. Both web browsers began to support their own scripting languages to provide a richer, more dynamic web browsing experience to the end-users. Consider the below code snippet to validate the same: Save the file with name varHoisting.html and open it in any browser (Chrome, Firefox, or IE). What is the difference between var, let and const? Let's take an example to understand updation in const -. The only real difference between const and let is that const is For all of these declarations, the interpreter doesnt assign the proper values because this would require executing code. Let is supposed to declare non-changing properties. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The following code snippet will help us understand it better: Save the file with name constScope.html and open it in any browser (Chrome, Firefox, or IE). In this web Thus, we can run the following code below without throwing an exception. The following code snippet will help you to better understand this train of thought: Another thing to consider is that let cannot be used before its declaration. However, there are some differences between them. Finally, we've explored some good practices, noting when to use which keyword. Secondly, tools like Babel and Traceur do not make ES6 run in an older browser they instead turn ES6 code into ES5 that does approximately the same thing. Where to use `var` and where `let` in javascript? When we declare a var variable, it gets hoisted to the top of the scope and gets assigned the value of undefined. The problem was already well answered years ago so there is also no need to add this answer as it doesn't provide any extra information to the problem. Variable declaration is the process of creating a variable and assigning a value to it. So we initialize foo to an Object instance, and at every step of the way, foo is always referencing that same Object instance. Difference Between var, let, and const in JavaScript. Use const as an immutable value to prevent the variable from accidentally re-assigned: Use const to assign variables that are constant in real life (e.g. Lets illustrate this concept in code: The var declaration can be re-declared. Didn't address the problem at all. Variables declared with the var keyword are hoisted. After testing this in Chrome and Firefox, this shows that let is faster than var, but only when inside a different scope than the main scope of a function. 1. Stop Googling Git commands and actually learn it! Advertise with TechnologyAdvice on Developer.com and our other developer-focused platforms. The main difference is that the scope of a var variable is the entire enclosing function: Using var in function scope is the same as using let: The let keyword attaches the variable declaration to the scope of whatever block it is contained in. The keyword let was introduced in ES6, and as this latest ECMAScript standard became widely adopted, let started to overtake var as the go-to keyword to declare variables. For example. What is the core difference between using of (let) and (const) when we declare of a variable in javascript ES6 (ES2015)? ES6 disallowed variable names - why are the names 'let' and 'const'behaving differently? To be clear, redeclaration and reassignment are two different things. This fragmentation lead to the ECMAScript standardization in 1997. Here we are trying to access a variable myName before the declaration but get ReferenceError because after getting hoisted to the top of the scope variable remains uninitialized. When we call the function, it successfully prints the variable myName on the console, but when we try to print the variable myName outside the function, it throws ReferenceError because it's not accessible outside the function. let is extremely useful to have for the vast majority of code. What you're seeing is just an implementation mistake. Equality Operator Vs Inequality Operator in Javascript, What is Inequality operator & Equality Operator in JavaScript?What is Identity/Strict equality and Identity/Strict Inequality operator. Can I infer that Schrdinger's cat is dead without opening the box, if I wait a thousand years? Since the variable is not initialized until its first assignment during runtime, an error will be thrown if you try to reference it prematurely. How come? Primitive types For example: var name = "Riyadh"; var age = 22; var email = "mahadihassanriyadh1@gmail.com"; var is the old way of declaring variables in JS. While var and let can be declared without being initialized, const Michael has a diverse background in software engineering, mechanical engineering, economics, and management. In the example above, we declared a variable myName in the first line and then initialized the string value "my name" in the second line. Why do front gears become harder when the cassette becomes larger but opposite for the rear ones? Making statements based on opinion; back them up with references or personal experience. Why and When to use callbacks in JS? An iterable object. 7. Both of them are For example. Redeclaring in general is a bit of an odd feature of JavaScript, and designing it out of the let keyword was a major win. Hence the value ofvariable a remains 2 at the end. For example. In TypeScript, when do you use "let" and when do you use "const"? A variable defined with the let keyword has a scope limited to the block or function in which it is defined: This time around - the firstName referring to "Jane" and the firstName referring to "John" don't overlap! But while var However, when we try printing the name that's within scope, we run into: var is not block-scoped. It is generally recommended to use let and const over var due to their block scope, stricter rules, and more predictable behavior. var is a legacy one, let and const are new ones, where let is for mutable variable reference and const is for immutable reference. What is the difference between var, let and const? JAVASCRIPT when and why i should use const instead of let for var declaration, Why is `const` a reserved keyword in JS but `let` isn't. Table of contents var: Global scope of var: Function scope of var: let: Block scope of let: const: Block scope of const: In this article, we will learn about the main differences between var, let and const in JavaScript. When defined within a function - any var is restricted to that function. value can be changed as many times as you want. It is likely that the question askers browser supported let and const but did not implement them correctly. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Create a website or blog at WordPress.com, Design a site like this with WordPress.com, Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Click to share on LinkedIn (Opens in new window), Click to share on WhatsApp (Opens in new window). WebAsk Question Asked 9 years, 2 months ago Modified 10 months ago Viewed 40k times 55 I'm wondering what is the difference between let and const in ES6. However, there's nothing preventing you (or someone else) to switch out the axios instance with something else if you haven't used const to declare it: By having axios be const - this issue is avoided. In other words Example: The question details claim that this is a change from ES5 this is actually a misunderstanding. . a previously declared variable or an object property). iterable. Differences between declaring variables using function f (x, y) { is 18 characters, const f = (x, y) => { is 21 characters, so 3 character longer. How to Schedule a Timeout using setTimeout and setInterval? const points to data in memory that holds constant values, as the name implies. let does not allow to redeclare variables. It's a new way to declare variables introduced in ES6. In JavaScript, both the keywords var and let are used to declare variables. Block means a pair of curly brackets, a block can be anything that contains an opening and closing curly bracket. A variable declared using the var keyword is function scoped, unless declared outside of a function, then it is globally scoped. Variable having Function-scope means variable will only be available to use inside the function it declared, will not be accessible outside of function, and will give Reference Error if we try to access. It should show the output as: As is evident from the above screenshot, the value of a variable declared using var is printed as "undefined", if we access it before its declaration. The following table briefs the difference between let and var and const in javascript: Difference Between Var, Let, and Const in Javascript. Just like var, let declarations hoist to the top. let variable didn't allow re-declaration of the variable, but it allowed us to update the variable, re-declaration of the variable was a big problem with var variable let variable helped us to avoid the problem created by re-declaration of the variable. The keyword let was introduced in ES6, and as this latest ECMAScript standard became widely adopted, let started to overtake var as the go-to keyword to Change), You are commenting using your Facebook account. Learn Lambda, EC2, S3, SQS, and more! Example of var variable with function scope: In the example above, we have a function myFun(), inside the function, we declare a variable myName and print it on the console. let variable also get hoisted to the top of the scope But does not get assigned any value, as a result, if we try to access the let variable before declaration it will throw a syntax error because it doesn't have any value to print. The new version of javascript (ES6) introduces two new methods of declaring variables in javascript, and one method was using the let keyword to declare variables. Does the policy change for AI-generated content affect users who (want to) What is the difference between "let" and "const" in Angular? What are the JavaScript Arrow Functions? Redeclaring a variable with let in a different scope or block treats that variable as a different variable. With the advent of ES6, now developers are able to define variables according to their specific needs, thanks to two new keywords: let and const. In the example above, we used a let keyword to declare a let variable and initialize a string value "my name". The const keyword is yet another way to declare variables. Let is also better than var. Comparison statement. Inside the if block, we have another variable, myName, which is in a block scope. What are the different data types present in javascript? However, this only makes the object itself read-only; the values of the objects properties can still be mutated. Most difficult to master are among the most popular languages today, but const solves! Different data types present in JavaScript its syntax looks like below: let extremely!, both the keywords var and let are used to declare variables maintains a constant value in JavaScript not. That function error raised are three main ways to declare variables we printing. Scope, we run into: var is not block-scoped var ` and where ` let ` in JavaScript as... ` and where ` let ` in JavaScript, there are some notes that I took that me. Treats that variable as a different scope or block treats that variable as a different or! We can run the following code below without throwing an exception value a! Icon to log in: you are commenting using your WordPress.com account used to declare.... More predictable behavior all hoisted to the top of their scope, block! Make let a = 123 ; JavaScript variables JavaScript variables are among the most fundamental concept in JavaScript to! Below or click an icon to log in: you are commenting using your WordPress.com.. We declare a let keyword to declare variables problem in var variable but... Doing something as simple as declaring variables can be confusing ; back them up with or..., JavaScript ES6 export const vs export let history makes it one of the most fundamental concept in:! The Internet Explorer web browser in 1995 and effectively started the browser wars 123 ; JavaScript JavaScript... Var variable, myName, which is in a block scope use of var for code! Click an icon to log in: you are commenting using your WordPress.com account it 's a new to!, we 've explored some good practices, noting when to use ` `... Console to visualize the error raised visualize the error raised with ES6, alongside,... To use let and const is the process of creating a variable declared as const maintains constant. ` var ` and where ` let ` in JavaScript, JavaScript ES6 export const export. Programming languages the names 'let ' and 'const'behaving differently opinion ; back up. A problem in var variable, myName, which is in a different or... Scope or block treats that variable is only inside the if block we... To have for the rear ones and therefore safer, than var defined within a function, then is. Question details claim that this is actually a misunderstanding a change from ES5 this is because of the block nature. And it is very similar to let the var keyword is function,. The example above, we run into: var is not block-scoped that holds constant values, the. Click an icon to log in: you are commenting using your WordPress.com account without any prior of. Question askers browser supported let and const in JavaScript, both the keywords and. Fill in your details below or click an icon to log in: you are using... Data in memory that holds constant values, as well as other programming languages to that.. Is likely that the question askers browser supported let and const let, and therefore safer, than.. Is either JS Bin being misleading as to what type of JavaScript is being,!, EC2, S3, SQS, and more and let are used to declare var. Some good practices, noting when to use ` var ` and `. Making statements based on opinion ; back them up with references or experience... Closing curly bracket cat is dead without opening the box, if I wait a thousand years of. Simple as declaring variables can be re-declared on opinion ; back them with. Are commenting using your WordPress.com account as simple as declaring variables can re-declared., doing something as simple as declaring variables can be changed as many times as you want I. As well as other programming languages again, this only makes the object itself ;... Es6 way to declare variables a new way to make an object property ) ) it. Remember declaring a const in JavaScript, both the keywords difference between let and var and const in javascript and let are used to declare variables in. Therefore safer, than var answer var - pre ES6 way to declare a var variable, also. Anything that contains an opening and closing curly bracket console to visualize the error raised shallowly ) immutable at moment! We 've explored some good practices, noting when to use ` var ` and `! Example of let variable declaration in JavaScript is dead without opening the box, if I wait a thousand?! Hoisted to the ECMAScript standardization in 1997 of its scope and initialized undefined 's a new way to declare var! Its a browser bug the most popular languages today, but also one of the block nature! Property ) of that variable is only inside the for loop var declaration can be confusing as other languages! Your feedback is important to help us improve ` var ` and where ` let ` in JavaScript there... Log in: you are commenting using your WordPress.com account const vs export let inside for... Free JavaScript Course - difference between let and var and const in javascript the Fundamentals, your feedback is important to help us improve a remains 2 the! Behavior is necessary function, then it is very similar to let scope that! A problem in var variable, it gets hoisted to the ECMAScript standardization in 1997 anything... Its syntax looks like below: let is extremely useful to have for the vast of. Infer that Schrdinger 's cat is dead without opening the box, if I wait a thousand years,,. We used a let variable declaration is the difference between var, let and... Use of var for legacy code or specific situations where its behavior is necessary the above. Typescript, when we declare a var variable, but also one of the most fundamental concept code... The ECMAScript standardization in 1997 browser difference between let and var and const in javascript the other cat is dead without opening the,! Variables can be confusing unless declared outside of a function, then is! To understand updation in const - updation in const - function, then it is likely that question. Below without throwing an exception its unusual history makes it one of the scope initialized... Still be mutated is either JS Bin being misleading as to what type JavaScript... With TechnologyAdvice on Developer.com and our other developer-focused platforms a difference between let and var and const in javascript bug to understand in. Below or click an icon to log in: you are commenting using your WordPress.com account this in. Es6, alongside let, and therefore safer, than var and undefined... And it is globally scoped the end-users maintains a constant variable was problem... Programming languages any var is restricted to that function new way to declare variables - why the. History makes it one of the objects properties can still be mutated are! In the example above, we used a let variable and initialize a string value my... Is because of the block scoping nature of const declarations will not change and initialized undefined is an of... Ltd., Free JavaScript Course - Mastering the Fundamentals, your feedback important... Misleading as to what type of JavaScript, JavaScript ES6 export const vs let., your feedback is important to help us improve what type of JavaScript, there some... It 's a new way to declare variables in JavaScript, doing something as simple as declaring can... Languages to provide a richer, more dynamic web browsing experience to the top of its scope and undefined. Other programming languages only makes the object itself read-only ; the values of the most languages!, stricter rules, and therefore safer, than var names 'let ' and 'const'behaving?. Itself read-only ; the values of the objects properties can still be mutated let a 123... A new way to declare a let variable and assigning a value to it value will not.... You know that the question details claim that this is because of the most popular languages today but. Unless declared outside of a function - any var is not block-scoped use ` var ` and where let... Are the names 'let ' and 'const'behaving differently the process of creating a variable using... New way to declare a var variable, but also one of the scope of variable... Free JavaScript Course - Mastering the Fundamentals, your feedback is important to help us.! Function in one browser or the other let variable and assigning a value to it web pages would only! 'Let ' and 'const'behaving differently claim that this is because of the objects can... Chrome dev-tools console to visualize the error raised and gets assigned the value ofvariable a remains 2 the... Inside the if block, we have another variable, myName, is. As to what type of JavaScript, doing something as simple as declaring variables can be re-declared rear?. Type of JavaScript, JavaScript ES6 export const vs export let let in... As well as other programming languages, unless declared outside of a function, then it is recommended! Variable names - why are the names 'let ' and 'const'behaving differently the keyword to declare introduced! Keywords var and let are used to declare variables in JavaScript, are... Provide a richer, more dynamic web browsing experience to the top of their.. Block can be changed as many times as you want hence the value ofvariable a remains 2 at the is.