What is a Variable?

A variable is a named container used to store data in a program. The stored value can be a number, string, object, array, or any other data type.

Example:

let name = "John";

let age = 25;

1. var

var is the oldest way to declare variables in JavaScript. It was used before the introduction of ES6 (ECMAScript 2015).

Syntax

var city = "Chennai";

Characteristics

• Function-scoped


• Can be redeclared


• Can be reassigned


• Hoisted and initialized with undefined
  

Example

var language = "JavaScript";

console.log(language);



language = "Python";

console.log(language);



var language = "Java";

console.log(language);

Output:

JavaScript

Python

Java

Scope Example

if (true) {

    var message = "Hello";

}



console.log(message);

Output:

Hello

Although message is declared inside the if block, it is still accessible outside because var is function-scoped, not block-scoped.

2. let

let was introduced in ES6 and is now the preferred way to declare variables whose values may change.

Syntax

let score = 90;

Characteristics

• Block-scoped


• Cannot be redeclared in the same scope


• Can be reassigned


• Hoisted but not initialized (Temporal Dead Zone)

Example

let marks = 80;



marks = 95;



console.log(marks);

Output:

95

Block Scope Example

if (true) {

    let number = 10;

    console.log(number);

}



console.log(number);

Output:

10

ReferenceError: number is not defined

The variable exists only inside the block where it is declared.

3. const

const is also introduced in ES6 and is used for values that should not be reassigned.

Syntax

const PI = 3.14159;

Characteristics

• Block-scoped


• Cannot be redeclared


• Cannot be reassigned


• Must be initialized during declaration

Example

const country = "India";



console.log(country);

Attempting to reassign it:

country = "USA";

Output:

TypeError: Assignment to constant variable.

Objects with const

A const object cannot be reassigned, but its properties can be modified.

const student = {

    name: "Alex",

    age: 20

};



student.age = 21;



console.log(student);

Output:

{

  name: "Alex",

  age: 21

}

Difference Between var, let, and const

Hoisting Example

console.log(a);

var a = 5;

Output:

undefined

console.log(b);

let b = 5;

Output:

ReferenceError

console.log(c);

const c = 5;

Output:

ReferenceError

When Should You Use Each?

Use const when:

• The variable value should not change.


• Declaring constants like API URLs or configuration values.

Example:

const company = "OpenAI";

Use let when:

• The value needs to change later.

Example:

let count = 0;



count++;



console.log(count);

Avoid var in modern JavaScript because:

• It ignores block scope.


• It allows accidental redeclaration.


• It can introduce bugs in large applications.

Best Practices

• Use const by default.


• Use let only when the value needs to change.


• Avoid using var in modern JavaScript development.


• Give variables meaningful names.


• Keep variable scope as small as possible.

Conclusion

Understanding the differences between var, let, and const is essential for writing reliable JavaScript code. While var was widely used in older JavaScript versions, modern development favors let and const because they provide block scope and help prevent common programming mistakes.

As a general rule:

• Use const for values that should remain unchanged.


• Use let for variables whose values will change.


• Avoid var unless you're maintaining legacy JavaScript code.