JavaScript const

The const keyword is used to declare constants in JavaScript. Constants are block-scoped, similar to variables declared using the let keyword, but their value cannot be reassigned.

1. Declaring Constants

You can declare a constant using the const keyword.

const pi = 3.14;
const gravity = 9.81;

console.log(pi); // Output: 3.14
console.log(gravity); // Output: 9.81

2. Block Scope

Constants declared with const have block scope, meaning they are only available within the block they are defined in.

if (true) {
    const blockScopedConst = "I am block scoped";
    console.log(blockScopedConst); // Output: I am block scoped
}
// console.log(blockScopedConst); // This will cause an error

3. Reassigning Constants

Constants declared with const cannot be reassigned. Attempting to do so will result in an error.

const pi = 3.14;
// pi = 3.14159; // This will cause an error

console.log(pi); // Output: 3.14

4. Mutable vs Immutable

While the value of a constant itself cannot be reassigned, the properties of objects and arrays declared with const can be changed.

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

person.age = 31; // This is allowed
console.log(person); // Output: { name: 'John', age: 31 }

// person = {}; // This will cause an error

const numbers = [1, 2, 3, 4, 5];

numbers.push(6); // This is allowed
console.log(numbers); // Output: [1, 2, 3, 4, 5, 6]

// numbers = [7, 8, 9]; // This will cause an error

5. Hoisting

Like variables declared with let, constants declared with const are not hoisted to the top of their block.

// console.log(hoistedConst); // This will cause an error
const hoistedConst = "This constant is not hoisted";

console.log(hoistedConst); // Output: This constant is not hoisted

6. Temporal Dead Zone

Similar to let, constants declared with const are subject to the temporal dead zone (TDZ), where accessing the constant before its declaration results in a ReferenceError.

function tdzExample() {
    // console.log(tempConst); // ReferenceError: Cannot access 'tempConst' before initialization
    const tempConst = "I am not accessible before my declaration";
    console.log(tempConst); // Output: I am not accessible before my declaration
}

tdzExample();

7. Best Practices

• Use const for variables that should not be reassigned.
• Use uppercase letters for constant names as a convention for values that are meant to be constants (e.g., const PI = 3.14;).
• Always declare constants at the top of their scope to avoid confusion.

By following these guidelines, you can write cleaner, more maintainable JavaScript code.