Creating JavaScript Arrays using Const

In 2015, JavaScript introduced the const keyword, which has since become a standard way to declare variables that should not be reassigned. This is especially useful when working with arrays.

const cars = ["Toyota", "Honda", "Ford"];
console.log(cars); // Output: ["Toyota", "Honda", "Ford"]

Cannot Reassign Arrays Declared with const

An array declared with const cannot be reassigned to a new array:

const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; // ERROR: Cannot reassign a const variable

This will result in a TypeError because const prevents reassigning the reference to a new array.

The const keyword does not make the array immutable; it only makes the reference constant. This means you can still modify the array's contents:

Elements Can Be Modified

You can update values and add new elements to a const array:

const cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Toyota"; // Modifies existing element
cars.push("Audi"); // Adds a new element
console.log(cars); // Output: ["Toyota", "Volvo", "BMW", "Audi"]

Array Must be Initialized

When declaring a const array, it must be initialized immediately:

const cars = ["Saab", "Volvo", "BMW"];
console.log(cars); // Output: ["Saab", "Volvo", "BMW"]

Declaring a const array without initializing it will cause an error:

const cars;
cars = ["Saab", "Volvo", "BMW"]; // ERROR: Missing initializer in const declaration

Scope of const Arrays

A const array has block scope, meaning it is only accessible within the block where it is defined.

const cars = ["Toyota", "Volvo", "BMW"];
console.log(cars[0]); // Output: "Toyota"
{
 const cars = ["Honda", "Volvo", "BMW"];
 console.log(cars[0]); // Output: "Honda"
}
console.log(cars[0]); // Output: "Toyota"

Redeclaring Arrays

Redeclaring an array using var is allowed:

var cars = ["Honda", "BMW"];
var cars = ["Toyota", "Ford"];
cars = ["Mazda", "Audi"];
console.log(cars); // Output: ["Mazda", "Audi"]

However, redeclaring or reassigning an array declared with const in the same scope is not allowed:

const cars = ["Toyota", "BMW"];
const cars = ["Toyota", "BMW"]; // ERROR: Identifier 'cars' has already been declared

Redeclaring in Different Blocks

Redeclaring an array with const in different blocks is permitted:

const cars = ["Volvo", "BMW"];
console.log(cars); // Output: ["Volvo", "BMW"]
{
 const cars = ["Toyota", "BMW"];
 console.log(cars); // Output: ["Toyota", "BMW"]
}
{
 const cars = ["Audi", "BMW"];
 console.log(cars); // Output: ["Audi", "BMW"]
}