Lesson 3.5. Variables in JavaScript

Like all programming languages, JavaScript allows you to use variables. variables are used to store numbers, text, lists ... Above all, we will see later it is possible to do operations with these variables.

There are three keywords for declaring variables:

the difference between var and let is the scope, i.e. the places of the code where the variables can be used.


The Var keyword is used to declare a global variable (for simplicity, it will be accessible everywhere). In the example below, we declare a variable x with an undefined value and a variable y equal to 5.

var x;
var y = 5;

In JavaScript, variables can contain different types of elements:

Here are some examples:

var integer = 12;
var real = 12.3456;
var text = "Hello Wolrd!";
var list = ["Jacques", "Nicolas", "François"];

The issue with variables created with var is that it is possible to create several times variables with the same name:

var myVariable = "Initial text";
var myVariable = "Something else";

// Display "Something else"
console.log (myVariable);

When working on large projects, this is a problem because it is easy to to declare an already existing variable, and thus to overwrite it. This is a source of bugs. To avoid this problem, it is better to use let.


To avoid creating two variables with the same name (which is possible with var), JavaScript introduced a new type in 2015: let.

The first advantage of let is that it is impossible to declare a variables twice:

let myVariable = 5;
let myVariable = 7;

The above code displays an error message in the console:

Uncaught SyntaxError: Identifier 'myVariable' has already been declared.

let a un autre avantage : la variable n'existe que dans le bloc où elle est déclarée, on dit que c'est une variable locale. Les variables locales permettent d'économiser la mémoire, car elles sont effacées à la fin du bloc dans lequel elles sont déclarées.

For these reasons, it is a better option to declare variables with let.


const is short for constant. It is not exactly a variable: variables are variable (they can change), as opposed to constants which are constant (their value will never change). The code below declares a constant, then tries to change its value:

const myVariable = 5;
myVariable = 12;

As you might expect, this code generates an error that states that you cannot change the value of a constant.

Uncaught TypeError: Assignment to constant variable.


Complete the script in the following code with the declaration of the variable message (with let). The message variable will contain the text The value of PI is:. Then declare a constant PI equal to 3.1415.


      // Create variables here

      console.log(message, PI);


What syntaxes are used to declare variables in JavaScript?

Check Bravo ! In the last case, the variable is declared, but its value is not defined. Try again...

What can be stored in a variable?

Check Bravo ! In JavaScript, variables are versatile. Try again...

What is the difference between var and let?

Check Bravo ! Les variables déclarées avec let occupe moins de place en mémoire et ne peuvent pas être déclarée plusieurs fois. Try again...

What is the difference between var and const?

Check Bravo! The variables can vary, the constants are fixed. Try again...

See also

Last update : 10/03/2022