In JavaScript, most of developers are well-known with const. It declares block-scoped local variables which means the value of a constant can't be changed through reassignment using the assignment operator.

However, have you defined the constant properly and clearly?

Naming Convention

According to Airbnb Naming Uppercase, exported constants should follow SCREAMING_SNAKE_CASE format, but not for constants within a file.

// Within a file
const helloWorldText = "Hello, World!"

// Exported constants
export const HELLO_WORLD_TEXT = "Hello, World!"

Examples

The following are examples of all exported data type constants:

// Number
const PI = 3.14159;

// String
const GREETING = "Hello, World!";

// Boolean
const IS_ACTIVE = true;

// Array
const COLORS = ["red", "green", "blue"];

// Object
const PERSON = {
    name: "John",
    age: 30
};

// RegExp
const EMAIL_PATTERN = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/;

// Null
const NO_VALUE = null;

// Undefined
const UNDEFINED_VALUE = undefined;

// Symbol
const UNIQUE_KEY = Symbol("unique");

Other uses

Besides defining data types, const is widely used in various other contexts.

a. Destructuring Arrays

const [a] = ["a"];

b. Destructuring Arrays with Default Values

const [a = 0] = [];

c. Destructuring Objects

const { a } = { a: "value" };

d. Destructuring Objects with Default Values

const { a = "default" } = {};

e. Global Constants

// Browser environment
window.GLOBAL_CONSTANT = "This is a global constant";

// Node.js environment
global.GLOBAL_CONSTANT = "This is a global constant";

f. Environment Variables (Node.js)

const PORT = process.env.PORT || 3000;

g. Template Literals with Constants

const name = "John";
const greeting = `Hello, ${name}!`;

h. Rest Parameters in Functions

function sum(...numbers) {
    const result = numbers.reduce((acc, num) => acc + num, 0);
    return result;
}

i. Destructuring Function Parameters

function printUser({ name, age }) {
    console.log(`Name: ${name}, Age: ${age}`);
}

j. Class Constants

class Circle {
    constructor(radius) {
        this.radius = radius;
    }

    getArea() {
        const PI = 3.14159;
        return PI * this.radius * this.radius;
    }
}

JavaScript CONSTANTS

Naming Convention

Examples

Other uses