In JavaScript, a variable is like a labeled box where you can store information (a value). To use these boxes, you must first "declare" them. Let's see how to do this with modern JavaScript tools.
let: The Reassignable Variable
let is the most common way to declare a variable whose value is likely to change.
You can do this in two steps: first the declaration, then the assignment.
// 1. Declaration of the variable "message"
let message;
// 2. Assignment of the value 'Hello World'
message = 'Hello World';
console.log(message); // Displays "Hello World"
To be more concise, you can declare and assign the value in a single line:
let message2 = 'Hello World';
console.log(message2); // Displays "Hello World"
const: The Immutable Constant
When you know that a value should never change, you use const. This is a good practice to make your code safer and more readable.
const monAnniversaire = '15.04.1990';
console.log(monAnniversaire); // Displays "15.04.1990"
A constant has strict rules:
- It must be initialized with a value during its declaration.
- You cannot change its value after its declaration.
// Trying to change the value will throw an error!
// monAnniversaire = '01.01.2000'; // TypeError: Assignment to constant variable.
Roughly, it's like let, but without the possibility of modification!
The Old Method: var
In older scripts, you will often see the keyword var.
var messageAncien = 'Hello World';
var works differently from let, especially in how the variable "exists" in your code (this is called the "scope"). By modern convention, we always prefer let and const. We will see the differences in more detail later.
Note on Readability
It is possible to declare multiple variables on a single line, but this is not recommended because it makes the code harder to read.
// Possible, but to be avoided
let prenom = 'Jean', age = 41, estMajeur = true;
Mastering let and const is the first essential step to writing modern and reliable JavaScript.