Javascript Const

JavaScript `const` (Constant Variables)

In JavaScript, const is used to declare constants, meaning the variable cannot be reassigned after being initialized. However, properties of objects and elements of arrays declared with const can still be modified.

🔹 Syntax of `const`

const variableName = value;

const → Declares the variable as a constant.
variableName → The name of the constant variable.
value → The initial value (must be assigned during declaration).

🔹 Key Features of `const`

1️⃣ Cannot Be Reassigned

Once a value is assigned to a const variable, it cannot be changed.

const PI = 3.14;
PI = 3.14159;  // ❌ Error: Assignment to constant variable

Unlike let or var, reassigning a const variable will cause an error.

2️⃣ Must Be Initialized at the Time of Declaration

You must assign a value when declaring a const variable; otherwise, an error occurs.

const x;  // ❌ Error: Missing initializer in const declaration

This is different from let and var, which allow declaration without initialization.

3️⃣ Block Scope

const has block-level scope, meaning it exists only inside the {} block where it is defined.

if (true) {
    const a = 10;
}
console.log(a); // ❌ Error: a is not defined

It does not leak outside the block, similar to let.

4️⃣ Works with Objects and Arrays

Although const prevents reassigning the variable itself, it does not prevent modifying properties of objects or changing elements in an array.

✅ Modifying Object Properties

const person = { name: "Alice", age: 25 };
person.age = 26;  // ✅ Allowed
console.log(person.age); // Output: 26

Here, we are not reassigning person, just modifying one of its properties.

✅ Modifying Array Elements

const numbers = [1, 2, 3];
numbers.push(4);  // ✅ Allowed
console.log(numbers); // Output: [1, 2, 3, 4]

We can modify an array's elements but cannot reassign the entire array:

numbers = [5, 6, 7];  // ❌ Error: Assignment to constant variable

🔹 When to Use `const`?

✔ Use const when the variable should never be reassigned after its initial declaration.
✔ Best for constants, objects, and arrays that don’t need reassignment.
✔ Improves code clarity and prevents accidental changes.

🔹 Conclusion

const prevents reassignment but allows modification of objects and arrays.
It provides better security by preventing unintended value changes.
Use const by default unless you need reassignment (then use let).

JAVASCRIPT