JavaScript CONSTANTS

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;
    }
}

Comments

[Eckehard]

Everybody can define it´s own naming conventions, so there is nothing wrong to do it differently, as long as you are not employed by AirBNB.

But although you mentioned this in a subordinate clause, it could be useful to go a bit more into detail about the strange behavoir of constants in Javascript, as they behave differently from constants in most other languages:

const c = [1, 2]  // defines a constant c
c[1] = 3 // this works, c = [1,3], value was changed
c = [3, 4] // this gives you an "Uncaught TypeError"
const [a,b] = [1,2] // defines 2 constants 
const {e,f} = [1,2] // -> e,f -> undefined, but no error
const {a,b} = {a:1,b:2} // a=1, b=2
const {e,f} = {a:1,b:2} // e,f=undefined, but no error
const h = {a,b} // h = {a: 1, b: 2}
const {i,j} = {1,2} // error: unexpected number
const l = (x) => console.log(x) // defines a function

[Muhammad A Faishal]

Yes, the naming convention is preference, but it's recommended to follow for constants.

I see, let me add some const implementations. Thanks for the feedback!

[Eckehard]

Great, this makes your post much more valuable, thank you!

There is one other strange thing about Javascript with reference to "const": While this is usable in normal Javascript, you cannot use it in object- or class variables. So, I suppose, your example j. is not correct, as this is a local constant, not a class constant. A class const was most useful, but it is not allowed:

class Circle {
    const PI = 3.14159; // --> Unexpected identifier 'PI'
    constructor(radius) {
        this.radius = radius;
    }

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

The same is true for object properties, that are always mutatable and cannot be defined using const, let or var. This is a very serious limitation of the JS class concept, as you cannot protect your internal variables against mutation.

[Jon Randy 🎖️]

...the value of a constant can't be changed through reassignment using the assignment operator, except Object and Array.

This is not correct, a const containing an Object or Array cannot be reassigned either. However, properties and contents of those types CAN be changed - but this is not reassignment.

[Eckehard]

You are right, but this is unexpected for users that came from other languages. The keyword "const" suggests, that the value cannot be mutated. So it is not obvious, why this is different:

const c = 5
const d = [5]
d = 6 // --> Error
d[0] = 6 // --> ok