let and const

In ES6 (ECMAScript 2015), let and const are two new ways to declare variables, alongside the traditional var. Here's a breakdown of each:

1. let:
   • Variables declared with let have block scope, which means they are only accessible within the block they are declared in (a block is defined by curly braces {}).
   • Variables declared with let can be reassigned.
   • Variables declared with let are not hoisted to the top of their scope.

Example:

// Using let to declare a variable
let x = 10;

if (true) {
    let y = 20;
    console.log(x); // Output: 10
    console.log(y); // Output: 20
}

console.log(x); // Output: 10
console.log(y); // Error: y is not defined (y is not accessible outside the if block)

1. const:
   • Variables declared with const also have block scope.
   • Variables declared with const cannot be reassigned.
   • However, for objects and arrays declared with const, their properties or elements can be modified.

Example:

// Using const to declare a constant
const PI = 3.14;

// PI = 3.1415; // Error: Assignment to constant variable.

const person = {
    name: 'John',
    age: 30
};

person.age = 31; // Valid: Modifying the properties of a constant object
console.log(person.age); // Output: 31

const numbers = [1, 2, 3];
numbers.push(4); // Valid: Modifying the elements of a constant array
console.log(numbers); // Output: [1, 2, 3, 4]

Using const is encouraged when you want to declare variables that should not be reassigned, as it helps in writing more predictable and maintainable code. Use let when you need to declare variables that may change their value.