Const vs. Let vs. Var in Javascript. Which one should you use?

Javascript is a powerful programming language that allows developers to create dynamic and interactive web applications. When it comes to declaring variables, developers often have a choice between using the let and var keywords. In this article, we will explore the differences between javascript let vs var, and which one is the best to use in different scenarios.

Javascript let

The let keyword was introduced in ECMAScript 6 (ES6) as a new way to declare variables in javascript. It is used to declare block-scoped variables, which means that the variable is only accessible within the block it was defined in. The block can be a loop, a function, or even just a set of curly braces.

For example, if we declare a variable using the let keyword inside a for loop, that variable will only be accessible inside that for loop, and not outside of it:

for (let i = 0; i < 5; i++) {

console.log(i); // 0, 1, 2, 3, 4

}

console.log(i); // Uncaught ReferenceError: i is not defined

In this example, the variable i is only accessible inside the for loop, and trying to access it outside of the loop will result in a ReferenceError.

Another benefit of using let is that it prevents variable hoisting, which is a behavior in javascript where variables declared with var are moved to the top of their scope. This can sometimes lead to unexpected results, especially in larger codebases.

Javascript var

The var keyword has been around since the early days of javascript and is used to declare variables with function-level scope. This means that the variable is accessible throughout the entire function in which it was defined, as well as any nested functions.

For example:

function example() {

var x = 10;

console.log(x); // 10



if (true) {

var y = 20;

console.log(y); // 20

}



console.log(y); // 20

}



example();

In this example, the variable x is accessible throughout the entire example() function, while the variable y is accessible within the if statement and also outside of it, due to var having function-level scope.

One downside to using var is that it can sometimes lead to unintended consequences, such as variable hoisting, where variables are moved to the top of their scope, potentially causing confusion or errors.

Const

This article will begin by beginning with const, which is likely to be one of the easiest to grasp and is the one I employ the most. Const first came into JavaScript after ES6 was released in mid-2015. It has revolutionized the way variables function since then.

In general, when you assign the variable’s value, you are employing const that means it is not possible to assign the variable another value later. That is, If you try to alter the value of a const variable, you’ll be greeted with an error. If you place the const declaration inside of the block, it will remain inside the block.

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Const is block-scoped. Const is a block-scoped object. It means that any action you accomplish using const will be limited to what you can do with it. Const variable is only employed inside the blocks of code that you first created. If you’re interested in learning what const can be used more in-depth, this video will do an excellent job of explaining how const functions and the definition of block scope in more detail.

Javascript let vs var: Which one should you use?

So, now that we understand the differences between let and var, which one should we use? As with most programming questions, the answer is “it depends”.

In general, it is recommended to use let over var, as let provides better scoping rules and can prevent potential issues caused by variable hoisting. Additionally, let is a more modern keyword and is more commonly used in modern javascript codebases.

However, there are still some cases where var may be the better choice. For example, if you are working with legacy code that uses var extensively, it may be easier to stick with var for consistency. Additionally, if you need to declare a variable that will be accessible throughout an entire function and any nested functions, var may be the better choice.

Best practices for using javascript let and var

Regardless of whether you choose to use let or var, there are some best practices you should follow to ensure your code is easy to read and maintain.

First, always declare your variables at the top of your functions or blocks. This makes it easier for other developers to understand the scope of the variable and can prevent issues caused by variable hoisting.

Second, try to use descriptive variable names that clearly indicate the purpose of the variable. This can prevent confusion and make your code easier to read.

Third, avoid using global variables whenever possible. Global variables can cause issues with naming conflicts and make it harder to reason about the behavior of your code. Instead, try to keep your variables as local as possible to the functions or blocks where they are needed.

Fourth, be mindful of variable reassignment. If you need to update the value of a variable, make sure you do so in a clear and intentional way, rather than accidentally overwriting its value.

Finally, consider using const in addition to let and var. const is another keyword introduced in ES6 that is used to declare variables that cannot be reassigned. This can prevent unintended changes to your variables and make your code more robust.

Difference between var and let in JavaScript

As we all know, to declare variables in Javascript, there are two options either declare using var or declare using let. The question is how to decide when to use var, and when to make use of let i.e what is the main distinctions between the two.

In the next section, we learn about the primary distinction between var and let in Javascript.

The primary distinction between let and var is that the scope of a variable that is defined using let is restricted to the block where it is declared, whereas a the variable defined with var has broad scope. Therefore, we can consider var to be the name of a word that defines a variable globally , regardless of the block’s scope.

The scope of let is not just restricted to the block within which it is defined , but variables with let do not appear in a windows that are global even though it is declared in a different block. However, we are able to access variables via var within a window object in the event that it is globally defined.

Because of their limited scope, variables are generally used in situations where there is a limited application of variables like loops and while loops, or within the limits of if conditions when var variables are used for situations when the value of the variable has to be stable and is used to access worldwide.

Another distinction between var and let is that variable defined with var could be changed to a different value, while variable cannot be redeclared when it’s defined using let.

FAQs JavaScript let vs var

Question: What is the difference between javascript let and var?

Answer: The main difference between let and var is their scoping rules. Variables declared with let have block-scoping, which means they are only accessible within the block they are declared in (e.g. within a loop or if statement). Variables declared with var have function-scoping, which means they are accessible throughout the entire function they are declared in.

Question: When should I use let vs var?

Answer: In general, it is recommended to use let over var because of its block-scoping and prevention of variable hoisting. However, there are still cases where var may be the better choice, such as when you need to declare a variable that is accessible throughout an entire function. Ultimately, the choice between let and var depends on the specific needs of your code and the scope you want your variables to have.

Question: What is variable hoisting?

Answer: Variable hoisting is a behavior in javascript where variable declarations are moved to the top of their respective scope, regardless of where the actual declaration appears in the code. This means that if you declare a variable with var inside a block, the variable will be accessible throughout the entire function, not just within the block. This can lead to unexpected behavior and make your code harder to reason about.

Question: Can I redeclare a variable with let or var?

Answer: You can redeclare a variable with var within the same function or block, but doing so can cause confusion and make your code harder to reason about. If you try to redeclare a variable with let within the same block, you will get a syntax error. It is generally recommended to avoid redeclaring variables and use unique variable names instead.

Question: What is the difference between const and let?

Answer: const is another keyword introduced in ES6 that is used to declare variables that cannot be reassigned. This can prevent unintended changes to your variables and make your code more robust. Variables declared with const also have block-scoping, like let. However, unlike let, const variables must be initialized with a value when they are declared, and cannot be left uninitialized.

Conclusion

In conclusion, javascript let and var are both useful keywords for declaring variables in javascript, but they have different scoping rules and behaviors. In general, it is recommended to use let over var for its block-scoping and prevention of variable hoisting. However, there are still cases where var may be the better choice, and both keywords should be used with best practices in mind to ensure readable, maintainable code. By understanding the differences between let and var and following best practices, developers can write more effective and robust javascript code.