JavaScript Variables and Data Types

Variables in JavaScript are used to store data that can be used and modified throughout a program. Each variable holds a specific data type, such as numbers, strings, or objects, that defines what kind of value it stores.

Declaring Variables in JavaScript

JavaScript allows you to declare variables using three keywords:

• var – (Old way) function-scoped
• let – block-scoped (preferred for modern JS)
• const – block-scoped and cannot be reassigned

Syntax:

Example:

Rules for Naming Variables

• Must begin with a letter, $, or _
• Case-sensitive (e.g., Name and name are different)
• Cannot use reserved keywords like let, class, function

JavaScript Data Types

JavaScript supports two categories of data types:

1. Primitive Data Types

These are basic and immutable:

• String
• Number
• Boolean
• Undefined
• Null
• Symbol (ES6+)
• BigInt (ES2020)

2. Non-Primitive (Reference) Data Types

These include more complex structures:

• Object
• Array
• Function

Example: Using Different Data Types

Key Takeaways

• Use let and const for declaring modern JavaScript variables
• JavaScript is dynamically typed—no need to specify the type explicitly
• Primitive types store simple data; reference types store complex structures
• Use typeof to check the type of a variable
• Choose the right data type based on the value your variable should hold