JavaScript const

Last Updated: 12 Feb, 2025

The const keyword in JavaScript is a modern way to declare variables, introduced in ECMAScript 6 (ES6). It is used to declare variables whose values need to remain constant throughout the lifetime of the application.

const is block-scoped, similar to let, and is useful for ensuring immutability in your code. Unlike let, the primary feature of const is that it cannot be reassigned once it has been initialized.

Syntax

const variable = value;

const is used to declare a variable and assign a value to it. After initialization, the value of a const variable cannot be reassigned.

Key Features of const

1. Block Scope

Variables declared with const are block-scoped, meaning they are accessible only within the block, statement, or expression in which they are defined.

if (true) {
    const x = 10;
    console.log(x); // Output: 10
}
console.log(x); // ReferenceError: x is not defined

2. No Reassignment

Variables declared with const cannot be reassigned after their initial declaration. Attempting to do so results in a TypeError.

const y = 20;
y = 30; // TypeError: Assignment to constant variable.

3. Must Be Initialized

Unlike let, a const variable must be initialized at the time of declaration. Declaring a const variable without assigning a value will throw a SyntaxError.

const z; // SyntaxError: Missing initializer in const declaration

4. Immutable Binding, Not Value

const makes the variable binding immutable, but if the value is an object or array, you can still modify its properties or contents.

const obj = { name: "Pranjal" };
obj.name = "Nanda";
console.log(obj.name); // Output: Nanda
 
const arr = [1, 2, 3];
arr.push(4);
console.log(arr); // Output: [1, 2, 3, 4]

5. No Redeclaration

Variables declared with const cannot be redeclared within the same scope, similar to let.

const a = 10;
const a = 20; // SyntaxError: Identifier 'a' has already been declared

6. Suitable for Constants

The const keyword is ideal for defining constants or variables whose values should not change during the program’s execution.

const PI = 3.14159;
const MAX_USERS = 100;
PI = 10; // TypeError: Assignment to constant variable.

7. Safer with Functions

Using const with functions ensures that the function reference cannot be reassigned, though the function itself can still be executed normally.

const greet = () => console.log("Hello, world!");
greet();
 
greet = () => console.log("Hi!"); // TypeError: Assignment to constant variable.

8. Integration with Modern JavaScript

const aligns with modern JavaScript practices, supporting features like destructuring and modules for cleaner, maintainable code.

const { name, age } = { name: "Meenal", age: 28 };
console.log(name, age); // Output: Meenal 28

Interesting Facts about const

• Block-scoped: Like let, const is block-scoped, meaning it is only accessible within the block, statement, or expression where it is defined.
• Immutable Binding: A const variable cannot be reassigned once it is initialized, providing a guarantee of immutability for the binding (not the value itself).
• Objects and Arrays: While the binding of a const variable is immutable, the contents of objects and arrays assigned to const can still be modified.
• Hoisting: Variables declared with const are hoisted to the top of their block but cannot be accessed until they are initialized, resulting in a “temporal dead zone” error if accessed before assignment.
• Used with Destructuring: const can be used in object and array destructuring, allowing you to easily unpack values from an object or array into individual variables.