Var Vs Let Javascript

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New features have been added to the JavaScript language with the release of ECMAScript6 (ES6). One of these characteristics affects the way we declare variables. With the addition of let and const, there were all kinds of questions about when to use each of the variable declarations.

In this article, we talk about each of the options, regarding their definition and use in the JavaScript environment.

Pre-ES6: Var

Prior to the release of ES6, the keyword used for variables was declaration of var .

Variables can also be redeclared and updated without errors

The first problem with var is obvious: redeclarations can cause bugs in your code if a variable has already been given the same name. There can be unintended consequences if something is declared again and you did not realize that the variable name has already been used in the codebase. Along with this, other peculiarities also arise regarding the scope and lifting.

Scope

The scope tells us what variables are available for us to use - we have variables that can be globally or locally scoped. What if we have the same variable declaration, but is global and the other is local?

When our var is declared outside the function, it is globally available to us globally. When "hello" is declared in the function, it is available only for the function - with functional or local scope

When we have a variable declaration which is the same, but the scope is different, the var in the function does not change the assignment of the global var, even with the invocation of helloWorld () . Since the var is declared in the function with the name "hello", it does not appear to be within the global reach of the alternate definition.

Hoist

When a variable is hoisted in JavaScript, the declaration of functions and variables is moved to the start of their scope before code execution. The variable is declared but not initialized, so the initial value of the variable is undefined.

Here, "hello" is hoisted and declared at the top of your reach. So basically the computer interprets the code like this:

The JavaScript engine sees that hello exists but does not know how hello is defined until the next step.

Eventually the developers who helped create the ECMAScript standard realized that there were some minor issues with var that could cost them time and money. So they decided to create more stringent keywords. This led to the creation of "leave" and "cost" in ES6

ES6:. Let

The let keyword is very similar to the var keyword in many ways. The main differences are the way errors are returned and the scope of each keyword is defined.

Variables declared and initialized with let keyword can be reassigned, but they cannot be redeclared

When you run the above code in a JavaScript console, the last line returns an error:

SyntaxError: The identifier ’hello’ has already been declared

Unlike var, declaring variables using let will not allow a declaration to the same. Variable name. You can, however, reassign if you wish, as shown in the example above.

Scope

Champ-wise, let is very similar to var. In addition to the rules that the global or local scope gives us, there is an additional constraint with let. Variables declared in a code block are only available for that code block. This is similar to functional scope, as functions are blocks of code that can be carried even further by having separate blocks within that function.

There are no errors in this code because the two instances of hello are treated as separate variables because they have different scopes.

Hoisting

Hoisting is another area where the let and var declarations are similar. "Var", when raised, is initialized as undefined. However, "leave" generates a reference error if you try to use it before it is initialized.

Note that the let keyword is the best choice in terms of syntax. This is because errors are generated when you can try to redeclare a variable or use it before the initialization process.

ES6: Const

The const keyword has more stringent guide lines than the let keyword < / em>. With const, variables cannot be declared or reassigned. A TypeError is generated if you try to reassign to a const.

That said, when working with objects or arrays, the properties of the object or array may be updated. As long as the basic structure of the object or table does not change, you can always update it.

As the example shows, reassigning the task to a new set of properties results in a TypeError: Assignment to constant variable. error. Const cannot be reassigned or redeclared, except in cases where you update the unique property in an object

Scope and lift

Just like the let keyword, const is scope block and it does not initialize not when hoisted, so it will throw an error if you try to use it before initialization.

The const keyword is ideal to use until you can ’t & rsquo; not the use more This is when you need to transfer or put to update your variable

Conclusion

In this article, we have looked at the differences between let, var and cost . Of all the keywords we currently have, it’s best not to use var unless necessary. Instead, use let or const.

👻 Read also: what is the best laptop for engineering students?

We hope this article has helped you to resolve the problem. Apart from Var Vs Let Javascript, check other __main__ Python module-related topics.

Want to excel in Python? See our review of the best Python online courses 2023. If you are interested in Data Science, check also how to learn programming in R.

By the way, this material is also available in other languages:



Julia Danburry

Moscow | 2023-02-01

Thanks for explaining! I was stuck with Var Vs Let Javascript for some hours, finally got it done 🤗. Checked yesterday, it works!

Anna Lehnman

California | 2023-02-01

Simply put and clear. Thank you for sharing. Var Vs Let Javascript and other issues with COM PHP module was always my weak point 😁. Checked yesterday, it works!

Ken Chamberlet

Milan | 2023-02-01

Python functions is always a bit confusing 😭 Var Vs Let Javascript is not the only problem I encountered. Will use it in my bachelor thesis

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