DEV Community

Cover image for JavaScript CONSTANTS
Muhammad A Faishal
Muhammad A Faishal

Posted on

JavaScript CONSTANTS

#javascript #node #frontend #backend

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!"
Enter fullscreen mode Exit fullscreen mode

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");
Enter fullscreen mode Exit fullscreen mode

Other uses

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

a. Destructuring Arrays

const [a] = ["a"];
Enter fullscreen mode Exit fullscreen mode

b. Destructuring Arrays with Default Values

const [a = 0] = [];
Enter fullscreen mode Exit fullscreen mode

c. Destructuring Objects

const { a } = { a: "value" };
Enter fullscreen mode Exit fullscreen mode

d. Destructuring Objects with Default Values

const { a = "default" } = {};
Enter fullscreen mode Exit fullscreen mode

e. Global Constants

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

// Node.js environment
global.GLOBAL_CONSTANT = "This is a global constant";
Enter fullscreen mode Exit fullscreen mode

f. Environment Variables (Node.js)

const PORT = process.env.PORT || 3000;
Enter fullscreen mode Exit fullscreen mode

g. Template Literals with Constants

const name = "John";
const greeting = `Hello, ${name}!`;
Enter fullscreen mode Exit fullscreen mode

h. Rest Parameters in Functions

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

i. Destructuring Function Parameters

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

j. Class Constants

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

    getArea() {
        const PI = 3.14159;
        return PI * this.radius * this.radius;
    }
}
Enter fullscreen mode Exit fullscreen mode

Top comments (5)

Collapse
 
efpage profile image
Eckehard • • Edited

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
Enter fullscreen mode Exit fullscreen mode
Collapse
 
maafaishal profile image
Muhammad A Faishal • • Edited

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!

Collapse
 
efpage profile image
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;
    }
}
Enter fullscreen mode Exit fullscreen mode

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.

Collapse
 
jonrandy profile image
Jon Randy 🎖️ • • Edited

...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.

Collapse
 
efpage profile image
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
Enter fullscreen mode Exit fullscreen mode

Read next

cat2d2 profile image

New open-source multiplayer canvas game!

kit2d2 -

niharikaa profile image

10 Advanced TypeScript Concepts Every Developer Should Know

Niharika Goulikar -

teo_nordic_d6988ee4c1221e profile image

Exploring Java's Role in Cloud Computing and AI for 2024

Teo Nordic -

guillaumesere profile image

Créer une Radio en Ligne avec l'API Radio Browser

Guillaume Sere -