JavaScript Const

The const keyword is used to declare variables whose binding should not be reassigned. In modern JavaScript, const is often the best first choice when you create a variable and do not plan to give that variable a completely new value later.

When to Use const in JavaScript

Use const when a variable should keep the same assigned reference after it is created. This makes your code more predictable and helps protect against accidental reassignment.

const siteName = "RedoHub";
const taxRate = 15;

console.log(siteName);

Here, siteName and taxRate are declared with const because they are meant to stay fixed.

const Must Be Assigned a Value

Unlike let, a const variable must receive a value at the time it is declared.

const country = "Bangladesh";

This is valid. But the following is not valid:

// const country; // Error

const Cannot Be Reassigned

Once a variable is declared with const, you cannot assign a completely new value to that variable name.

const pi = 3.1416;
// pi = 3.14; // Error

This rule makes const a strong choice for values that should stay stable throughout the program.

Block Scope of const

Like let, const is block-scoped. A variable declared inside a block exists only inside that block.

if (true) {
    const message = "Welcome";
    console.log(message);
}

// console.log(message); // Error

This means message is only available inside the if block.

const and Arrays

A common beginner confusion is this: const does not mean the contents of an array can never change. It means the variable itself cannot be reassigned to a different array.

const colors = ["red", "green", "blue"];

colors[0] = "yellow";
colors.push("black");

console.log(colors);

This works because the array itself still exists under the same variable name. You are changing the contents, not reassigning the variable.

const and Objects

The same idea applies to objects. A const object can have its properties changed, but the variable cannot be reassigned to a different object.

const user = {
    name: "Nabila",
    age: 22
};

user.age = 23;

console.log(user);

const vs let

Both const and let are modern ways to declare variables, but they are used in different situations.

Best Practice

Many developers prefer to use const by default and switch to let only when they know a variable will change. This creates safer and more intentional code.

Key Points to Remember

• const is used for variables that should not be reassigned
• A const variable must be assigned a value immediately
• const is block-scoped, just like let
• You cannot reassign a const variable to a new value
• Arrays and objects declared with const can still have their contents changed
• Use const by default when the variable is meant to stay stable