Lesson 3: JavaScript Data Types

Learning Objectives

By the end of this lesson, learners will:

• Understand what data types are and why they matter in programming
• Learn about JavaScript’s primitive and non-primitive data types
• Be able to declare and use different data types
• Recognize the difference between undefined and null
• Use the typeof operator to check data types

What Are Data Types?

Every piece of data in a JavaScript program has a type.
A data type tells JavaScript:

• What kind of value it is
• What you can do with it

📦 Think of it like different containers:

• A bottle for water (number)
• A folder for documents (string)
• A switch for lights (boolean)

Categories of Data Types

JavaScript has two major categories of data types:

1. Primitive Data Types

These are the most basic values in JavaScript. They are immutable (cannot be changed).

2. Non-Primitive Data Types (Objects)

These store collections of data or more complex entities.

We’ll explore objects, arrays, and functions in later modules.

Primitive Data Types Explained

1. Strings

• Represent text and are wrapped in quotes.
• Can use single ('), double ("), or template literals (`).

📌 Example:

let name = "Alice";
let greeting = `Hello, ${name}!`; // using template literals

2. Numbers

• Can be whole numbers or decimals.
• JavaScript uses only one number type (no distinction between integers and floats).

📌 Example:

let age = 30;
let price = 19.99;

3. Booleans

• Only two possible values: true or false.
• Used for decision-making (conditions, comparisons).

📌 Example:

let isStudent = true;
let passed = false;

4. Undefined

• A variable that’s declared but not assigned a value.

📌 Example:

let x;
console.log(x); // undefined

5. Null

• A variable with a deliberate empty value.
• Used to “clear out” or “reset” a variable.

📌 Example:

let user = null;

🔍 undefined means “not assigned.”
🔍 null means “intentionally empty.”

6. BigInt (Advanced Use)

• Used to store numbers larger than JavaScript’s safe limit.
• Add n to the end of a number.

📌 Example:

let big = 123456789012345678901234567890n;

7. Symbol (Advanced Use)

• A unique, immutable value used mostly in advanced JavaScript features like object keys.

📌 Example:

let id = Symbol("id");

Using the typeof Operator

Use the typeof keyword to check a variable’s type:

📌 Example:

let name = "Alice";
console.log(typeof name); // "string"

let age = 30;
console.log(typeof age); // "number"

let isOnline = true;
console.log(typeof isOnline); // "boolean"

Practice Examples

let city = "Berlin";        // String
let temperature = 22.5;     // Number
let isSunny = true;         // Boolean
let nothingHere = null;     // Null
let notSet;                 // Undefined
let largeNumber = 9007199254740991n; // BigInt

Summary Table