Understanding Variables and Data Types in JavaScript

Source: Dev.to

Think of variables as boxes that store information. If you have multiple boxes, you need to label each one to understand what is stored inside.

In programming, a variable stores data and the variable name is the label.

let name = "John";

Here name is the label and "John" is the value.

How to declare variables

You can use let, var, and const to declare variables:

let age = 25;
const country = "USA";
var isStudent = true;

Primitive data types

String

let name = "Alice";

Number

let age = 20;

Boolean

let isStudent = true;

Null

let middleName = null;

Undefined

let score;

When to use var, let, & const

• var – declares function‑scoped or globally‑scoped variables.
• let – declares a block‑scoped variable.
• const – declares a block‑scoped variable whose value cannot be reassigned.

What is block scope

Scope determines where a variable can be accessed. Think of it like placing a box in different rooms:

• A box in the living room (global scope) can be seen by everyone.
• A box in your bedroom (block scope) can only be seen by you.

{
  let message = "Hello";
  console.log(message); // ✅ Works here
}

console.log(message); // ❌ Error (not accessible outside)

Declare variables

let name = "Emma";
let age = 22;
let isStudent = true;

Try changing values

age = 23;           // ✅ Allowed
isStudent = false; // ✅ Allowed

Now try with const

const country = "Canada";
country = "USA"; // ❌ Error: Assignment to constant variable

Scope visualization

Global Scope
  ├── name
  ├── age
  └── Block Scope
        └── message