JavaScript Variables - JavaScript Tutorial

What are Variables?

Variables are used to store data that can be referenced and manipulated in a program. They act as containers for storing information. In JavaScript, you can declare variables using var, let, or const.

Using var

The var keyword is used to declare a variable, which can be reassigned later and has function scope.

var name = "John";
console.log(name);  // Output: John

JavaScript
​x
 
var name = "John";console.log(name);  // Output: John

Using let

The let keyword is used to declare a block-scoped variable, which can be reassigned.

let age = 30;
console.log(age);  // Output: 30

JavaScript
 
let age = 30;console.log(age);  // Output: 30

Using const

The const keyword is used to declare a block-scoped variable that cannot be reassigned.

const PI = 3.14;
console.log(PI);  // Output: 3.14

JavaScript
 
const PI = 3.14;console.log(PI);  // Output: 3.14

Variable Naming Rules

Variable names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter, underscore, or dollar sign.
Variable names are case-sensitive (myVar and myvar are different).
Reserved words (like let, class, return, and function) cannot be used as variable names.

Initializing Variables

Variables can be declared without being assigned a value. In such cases, they are initialized with the value undefined.

let city;
console.log(city);  // Output: undefined
city = "New York";
console.log(city);  // Output: New York

JavaScript
 
let city;console.log(city);  // Output: undefinedcity = "New York";console.log(city);  // Output: New York

Reassigning Variables

Variables declared with var or let can be reassigned new values.

let score = 50;
console.log(score);  // Output: 50
score = 100;
console.log(score);  // Output: 100

JavaScript
 
let score = 50;console.log(score);  // Output: 50score = 100;console.log(score);  // Output: 100

JavaScript Variables Example Code

Explanation of Code:

Using var: Declares a variable name and assigns it a value of “John”.
Using let: Declares a variable age and assigns it a value of 30, which is later updated to 31.
Using const: Declares a constant variable PI and assigns it a value of 3.14.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JavaScript Variables Example</title>
</head>
<body>
    <script>
        // Using var
        var name = "John";
        console.log("Name: " + name);

        // Using let
        let age = 30;
        console.log("Age: " + age);

        // Using const
        const PI = 3.14;
        console.log("PI: " + PI);

        // Reassigning let variable
        age = 31;
        console.log("Updated Age: " + age);
    </script>
</body>
</html>

HTML 
 
<!DOCTYPE html><html lang="en"><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, initial-scale=1.0">    <title>JavaScript Variables Example</title></head><body>    <script>        // Using var        var name = "John";        console.log("Name: " + name);​        // Using let        let age = 30;        console.log("Age: " + age);​        // Using const        const PI = 3.14;        console.log("PI: " + PI);​        // Reassigning let variable        age = 31;        console.log("Updated Age: " + age);    </script></body></html>