JavaScript Data Types

In the previous lesson, we learned how to declare variables. Now let’s look at the different kinds of values, called data types, that those variables can hold.

🔢 What are Data Types?

A data type describes the kind of value you are working with. A piece of text and a number are different types, and JavaScript treats them differently.

JavaScript values fall into two groups:

1. Primitive types 🧱 - simple, single values like text, numbers, and true/false
2. Objects 📦 - collections of values, like arrays and objects

🧱 Primitive Data Types

These are the basic building blocks. Here are the primitive types you will use most:

Here they are in code:

const name = "Alex"; // String
const age = 25; // Number
const isStudent = true; // Boolean
let address; // Undefined (no value assigned)
const car = null; // Null (intentionally empty)

Let’s walk through each line to see which type we created:

• name holds text inside quotes, so it is a string.
• age holds a plain number with no quotes, so it is a number.
• isStudent holds true, so it is a boolean.
• address is declared but never given a value, so it is undefined.
• car is set to null, which means we deliberately marked it as empty.

📦 Objects

When you need to group related values together, you use an object or an array. These are not primitive types.

// An object: a collection of named values
const user = {
  name: "Alex",
  age: 25,
};

// An array: an ordered list of values
const colors = ["red", "green", "blue"];

Here we group several values into two containers:

• user is an object written with curly braces {}, where each value has a name, called a key. It stores name set to "Alex" and age set to 25.
• colors is an array written with square brackets [], where values sit in order. It holds three strings: "red", "green", and "blue".

We will cover objects and arrays in detail in later lessons.

🔍 Checking the Type With typeof

You can check the type of any value using the typeof operator. It returns the type as a string.

console.log(typeof "Hello"); // "string"
console.log(typeof 42); // "number"
console.log(typeof true); // "boolean"
console.log(typeof undefined); // "undefined"

Each line asks typeof to report the type of a value, and console.log prints the answer:

• typeof "Hello" returns "string" because the value is text.
• typeof 42 returns "number" because the value is a number.
• typeof true returns "boolean" because the value is true or false.
• typeof undefined returns "undefined" because the value has no assigned value.

⚠️ Common Mistakes to Avoid

🔧 Try It Yourself!

🧩 What You’ve Learned

• ✅ A data type describes the kind of value you are working with
• ✅ Primitive types include string, number, boolean, undefined, and null
• ✅ Objects and arrays group multiple values together
• ✅ undefined means “no value yet”; null means “empty on purpose”
• ✅ typeof tells you the type of a value

🚀 What’s Next?

Now that you know the data types, we will learn how to convert values from one type to another. Let’s continue to Type Conversion.