JavaScript Constant Variables: Immutable Foundations for Stable Code

A constant variable, as the name suggests, is a variable that holds a value that cannot be changed once it is assigned.

In JavaScript, constant variable are declared using the ‘const’ keyword.

Declaration and initialization

const pi = 3.14159;

Constant must be assigned a value at the time of declaration. Unlike regular variable, which can be declared without initialization, constant variables require an immediate value assignment.

Basic const usage

Constant can be decalared with uppercase or lowercase, but a common convention is to use all-uppercase letters.

// define MY_FAV as a constant and give it the value 7
const MY_FAV = 7;

// this throws an error
// Uncaught TypeError: Assignment to constant variable
MY_FAV = 20;

// MY_FAV is 7
console.log("my favorite number is: " + MY_FAV);

// trying to redeclare a constant throws an error
// Uncaught SyntaxError: Identifier 'MY_FAV' has already been declared
const MY_FAV = 20;

// this throws an error too
var MY_FAV = 20;

// this throws an error too
let MY_FAV = 20;

Block Scoping

const x = 1;
function exampleFunction() {
  if (true) {
    const x = 10;
    console.log(x); // Output: 10
  }
  console.log(x); // Output: 1
}

exampleFunction();

In this example, the ‘const’ variable ‘x’ is declared 2 times outside function and inside function so it output 10 inside if block and output 1 outside if block.

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

const person = { name: 'John', age: 30 };
person.age = 31;
console.log(person); // Output: { name: 'John', age: 31 }