A constant in JavaScript is a variable that cannot be reassigned after its initialization.
This means that once a value has been assigned to a constant, that value cannot be changed during the execution of the program. This prevents potential mistakes and improves code maintainability.
In JavaScript, constants are declared using the const
keyword, which was introduced in ECMAScript 6 (2015).
If you want to learn more about Constants
check out the Introduction to Programming Course read more
Declaring constants
The basic syntax for declaring a constant in JavaScript is as follows:
const CONSTANT_NAME = value;
For example,
const PI = 3.14159;
const API_URL = "https://api.example.com/data";
In these examples,
PI
andAPI_URL
are constants- If we try to reassign a new value to these constants, it will generate an error
Using constants with primitive types
Constants can be used with any primitive type in JavaScript (such as numbers, strings, booleans).
const MAX_USERS = 100;
const WELCOME_MESSAGE = "Welcome to the JavaScript course";
const IS_ADMIN = true;
Using constants with objects and Arrays
Although its name can be misleading, do not confuse constant with immutable. const
ensures that the value of a variable cannot be changed after its assignment.
However, if the variable is a reference (like an object or an array) the properties or elements of that object or array can be modified.
Let’s see it with an example,
const user = {
name: "Juan",
age: 30
};
// Trying to reassign the object
user = { name: "María", age: 25 }; // This will generate an error
// Modifying a property
user.age = 31; // This is valid
In the example,
- We CANNOT reassign the
user
object because it is constant - But NOTHING PREVENTS us from modifying its members.
Similarly, in an array
const numbers = [1, 2, 3, 4, 5];
// Trying to reassign the array
numbers = [10, 20, 30]; // This will generate an error
// Modifying an element
numbers[0] = 10; // This is valid
// Adding a new element
numbers.push(6); // This is valid
- We CANNOT reassign the
numbers
object - But NOTHING PREVENTS us from modifying its elements
This is how references work in programming
if you have questions, check the Introduction to Programming Course read more ⯈
Constants in the block context
Constants, like variables declared with let
, have block scope. This means that a constant is only available within the block in which it is declared.
if (true) {
const MESSAGE = "Hello World";
console.log(MESSAGE); // "Hello World"
}
console.log(MESSAGE); // Error: MESSAGE is not defined
Additionally, they are also subject to the TDZ (Temporal Dead Zone). This means that they cannot be accessed before their declaration in the code.
console.log(GREETING); // Error: Cannot access 'GREETING' before initialization
const GREETING = "Hello";