Let and Const

In JavaScript, there are multiple ways to declare variables, and each way has its own characteristics and use cases. The let and const keywords were introduced in ES6 as alternatives to the traditional var keyword. Let's explore the differences between them and when to use each one:

Variable Declaration

Variables declared with the var keyword are function-scoped. This means that they are accessible anywhere within the function in which they are declared. On the other hand, variables declared with the let and const keywords are block-scoped. This means that they are accessible only within the block of code in which they are defined.

Variable Reassignment

Variables declared with var can be reassigned multiple times without any restrictions. However, variables declared with let can be reassigned within their block scope, while variables declared with const cannot be reassigned once they have been assigned a value. Note that for objects and arrays declared with const, their properties or elements can still be modified.

Best Practices

• Use var when you need to declare a variable with function scope and want to allow reassignment.
• Use let when you need to declare a variable with block scope and may need to reassign its value.
• Use const when you need to declare a variable with block scope and want to enforce immutability.

1// Variables declared with `var`
2
3// The variable `name` is declared with `var` keyword
4var name = 'John';
5
6// The variable `age` is also declared with `var` keyword
7var age = 30;
8
9// Variables declared with `let`
10
11// The variable `city` is declared with `let` keyword
12let city = 'New York';
13
14// The variable `year` is also declared with `let` keyword
15let year = 2022;
16
17// Variables declared with `const`
18
19// The variable `country` is declared with `const` keyword
20const country = 'USA';
21
22// The variable `month` is also declared with `const` keyword
23const month = 'January';

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// Variables declared with `var`

​

// The variable `name` is declared with `var` keyword

var name = 'John';

​

// The variable `age` is also declared with `var` keyword

var age = 30;

​

// Variables declared with `let`

​

// The variable `city` is declared with `let` keyword

let city = 'New York';

​

// The variable `year` is also declared with `let` keyword

let year = 2022;

​

// Variables declared with `const`

​

// The variable `country` is declared with `const` keyword

const country = 'USA';

​

// The variable `month` is also declared with `const` keyword

const month = 'January';