let name = "John";

Here:

• let → keyword used to declare a variable

• name → variable name

• "John" → stored value

You can later use the variable in your program.

console.log(name);

Output:

John

1. var in JavaScript

var is the old way to declare variables in JavaScript.

Example:

var age = 25;
console.log(age);

Output:

25

Characteristics of var

• Function scoped

• Can be redeclared

• Can be updated

Example:

var city = "London";
var city = "Paris";

console.log(city);

Output:

Paris

Because of these behaviors, var can sometimes cause unexpected bugs.

2. let in JavaScript

let was introduced in ES6 (2015) and is now widely used.

Example:

let age = 20;
age = 25;

console.log(age);

Output:

25

Characteristics of let

• Block scoped

• Can be updated

• Cannot be redeclared in the same block

Example:

let name = "Alex";
let name = "John"; // Error

This makes let safer than var.

3. const in JavaScript

const is used to declare constant variables.

Example:

const pi = 3.14;
console.log(pi);

Output:

3.14

Characteristics of const

• Block scoped

• Cannot be updated

• Cannot be redeclared

Example:

const age = 30;
age = 40; // Error

Block Scope Example

Block scope means the variable only exists inside the block {}.

Example:

{
let number = 10;
console.log(number);
}

console.log(number); // Error

The variable cannot be accessed outside the block.

Comparison Table

| Feature    | var            | let   | const |
| ---------- | -------------- | ----- | ----- |
| Scope      | Function       | Block | Block |
| Redeclare  | Yes            | No    | No    |
| Update     | Yes            | Yes   | No    |
| Introduced | Old JavaScript | ES6   | ES6   |

When to Use var, let, and const

Use const

When the value should not change.

const country = "USA";

Use let

When the value may change later.

let score = 10;
score = 20;

Avoid var

In modern JavaScript, developers usually avoid using var.

Example Program

const name = "Emma";
let age = 25;

console.log("Name:", name);
console.log("Age:", age);

Output:

Name: Emma
Age: 25

Common Beginner Mistake

Trying to change a const variable.

const price = 100;
price = 200; // Error

Always remember:

const variables cannot be reassigned.

Summary

In this tutorial, you learned:

• What variables are in JavaScript

• How to use var, let, and const

• Differences between them

• When to use each one

Key takeaway:

• Use const by default

• Use let when values change

• Avoid var

What’s Next?

In the next tutorial, you will learn:

JavaScript Data Types Explained with Examples.