The const keyword in JavaScript is used to declare constant variables. These variables cannot be reassigned once they are defined, making them ideal for values that should remain constant throughout the program.

Syntax of const

const variableName = value;

• const: The keyword to declare a constant variable.
• variableName: The name of the constant variable.
• value: The value assigned to the variable (required).

Features of const

Block Scope
const variables are confined to the block in which they are declared.
Example:

{
    const pi = 3.14;
    console.log(pi); // Outputs: 3.14
}
console.log(pi); // Error: pi is not defined

No Reassignment
You cannot reassign a new value to a const variable after it has been declared.
Example:

const name = "Junaid";
name = "Arsheen"; // Error: Assignment to constant variable

Must Be Initialized
const variables must be initialized at the time of declaration.
Example:

const age; // Error: Missing initializer in const declaration

Works with Objects and Arrays
While you cannot reassign an object or array declared with const, you can modify their contents.
Example:

const person = { name: "Junaid" };
person.age = 29; // Adding a new property is allowed
console.log(person); // Outputs: { name: "Junaid", age: 29}

const numbers = [1, 2, 3];
numbers.push(4); // Modifying the array is allowed
console.log(numbers); // Outputs: [1, 2, 3, 4]

Difference Between const, let, and var

The const keyword is an essential tool in JavaScript for declaring immutable variables. It enhances code reliability, reduces errors, and is especially useful for objects and arrays. By understanding and leveraging const, you can write cleaner, more maintainable code.