JavaScript constants, declared using the const keyword (introduced in ES6), are variables whose values cannot be reassigned after initialization. They provide a mechanism to ensure data integrity and prevent accidental modifications.

Declaring Constants:

The syntax for declaring a constant is straightforward:

JavaScript

const constantName = value;

• constantName: Choose a meaningful name that reflects the constant's purpose.

• value: The initial value assigned to the constant.

Key Points:

• Initialization is Mandatory: You must assign a value to a constant during its declaration.

• Immutable Reference: While you can't reassign the constant itself, you can modify the contents of arrays or objects referenced by a constant (explained later).

Example:

JavaScript

const PI = 3.14159;
const MAX_VALUE = 100;

console.log("PI:", PI);   // Output: PI: 3.14159

Block Scope:

Like let, constants have block scope. This means they are only accessible within the code block where they are declared (e.g., if statement, loop, or function body).

JavaScript

{
  const x = "John";
}

// x is not accessible here

Redeclaration and Reassignment:

• Redeclaration: You cannot redeclare a constant with the same name within the same block.

• Reassignment: Once assigned, a constant's value cannot be changed using the assignment operator (=).

Constant Arrays and Objects:

While you cannot reassign the reference of a constant array or object, you can modify their contents:

JavaScript

const fruits = ["apple", "banana", "orange"];

fruits[0] = "mango"; // Update element at index 0
fruits.push("grape");  // Add element to the end

console.log(fruits); // Output: ["mango", "banana", "orange", "grape"]

Constant vs. Let:

Best Practices:

• Use const by default for variables that should remain constant throughout your program.

• Choose clear and descriptive names for constants.

• If you need to modify the contents of a collection (array or object), consider using let to declare the variable referencing the collection.

By effectively using constants, you can write more reliable and predictable JavaScript code.