In JavaScript, every value has a type. A data type tells you what kind of information that value represents. For example, some values are 

), and some represent missing or unknown values (

Think of a data type as a label on a wire that tells you what kind of thing it is pointing to.

Numbers are values that represent numerical quantities. They can be integers like 42 or decimals like 3.14.

Strings represent text. They are enclosed in quotes (single or double).

Booleans are simple values that can only be either 

. They’re useful for making decisions in your code.

When a variable has not been assigned any value, its value is undefined.

 is a special value that represents an intentional absence of any object value.

. This is a quirk in JavaScript due to historical reasons and you don't have to worry about it. We'll get to 

 operator to see what type a value is. It’s a helpful tool when you’re learning how JavaScript categorizes different values.

Print the value of num and its type using console.log.

index.js

index.test.js

Print a Number and Check Its Type

 and assign it the value "Hello, world!".

Print the value of greeting and its type using console.log.

Print a String and Check Its Type

Print value and its type using console.log.

Print the new value of value and its type using console.log.

Working with Undefined and Null

## What Are Data Types?

In JavaScript, every value has a type. A data type tells you what kind of information that value represents. For example, some values are **numbers**, others are text (**strings**), some are true/false values (**booleans**), and some represent missing or unknown values (**undefined** and **null**).

Think of a data type as a label on a wire that tells you what kind of thing it is pointing to.

---

## Numbers

Numbers are values that represent numerical quantities. They can be integers like 42 or decimals like 3.14.

```javascript
let num = 42;
console.log(num);           // prints 42
console.log(typeof num);    // prints "number"
```

---

## Strings

Strings represent text. They are enclosed in quotes (single or double).

```javascript
let greeting = "Hello, world!";
console.log(greeting);           // prints "Hello, world!"
console.log(typeof greeting);    // prints "string"
```

---

## Booleans

Booleans are simple values that can only be either **true** or **false**. They’re useful for making decisions in your code.

```javascript
let isSunny = true;
console.log(isSunny);           // prints true
console.log(typeof isSunny);    // prints "boolean"
```

---

## Undefined and Null

### Undefined

When a variable has not been assigned any value, its value is undefined.

```javascript
let value;
console.log(value);           // prints undefined
console.log(typeof value);    // prints "undefined"
```

### Null

**null** is a special value that represents an intentional absence of any object value.

☝️Although **null** means “nothing,” note that **typeof null **returns **"object"**. This is a quirk in JavaScript due to historical reasons and you don't have to worry about it. We'll get to **object** later on in the course.

```javascript
let empty = null;
console.log(empty);           // prints null
console.log(typeof empty);    // prints "object"
```

---

## The typeof Operator

You can use the **typeof** operator to see what type a value is. It’s a helpful tool when you’re learning how JavaScript categorizes different values.

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

---

## Exercises

@exercise 0ab8d86206cb

@exercise 32c7ef852310

@exercise ef88640a71a5


- Luo muuttuja nimeltä num käyttäen **let** ja anna sille arvoksi 42.
- Tulosta num:n arvo ja tyyppi käyttäen console.log.

Tulosta luku ja tarkista sen tyyppi

- Luo muuttuja nimeltä tervehdys käyttäen **let** ja anna sille arvoksi "Hei, maailma!".
- Tulosta tervehdyksen arvo ja tyyppi käyttäen console.log.

Tulosta merkkijono ja tarkista sen tyyppi

- Määritä muuttuja nimeltä value käyttäen **let**-avainsanaa, mutta älä määritä sille lainkaan arvoa.
- Tulosta muuttujan arvo ja tyyppi käyttäen console.log-komentoa.
- Sitten anna muuttujalle arvoksi null.
- Tulosta muuttujan uusi arvo ja tyyppi käyttäen console.log-komentoa.

Työskentelyä Undefinedin ja Nullin kanssa

Data Types

Learn programming with JavaScript, arguably the most important and versatile programming language in today's world, at least for web development.

Welcome to Hakatemia’s beginner-friendly JavaScript course, where you will learn the fundamentals of programming with JavaScript. In today’s world, JavaScript is one of the most popular and versatile programming languages—especially in web development, where it holds a unique position: no other programming language can run both in the browser and on the server. For this reason, JavaScript has become extremely popular and is used for developing user interfaces (React, Vue, Angular, Svelte, etc.), back-end systems (NodeJS/Express, HAPI, etc.), and full-stack solutions (NextJS, NuxtJS, etc.).

These JavaScript frameworks will be covered in later Hakatemia courses, but before we get there, we must first master the basics of JavaScript programming. This course is designed precisely for that purpose.

Throughout the course, you’ll learn core programming concepts such as variables, operators, conditional statements, functions, arrays, and objects. Each module is divided into small, manageable segments with plenty of practical exercises.

This course is being released as a beta—the first part of Hakatemia’s comprehensive web development curriculum—and it is the first course that uses Hakatemia’s new programming exercise environment. Please provide feedback using the button at the bottom of the page or on Discord if you encounter any issues!

Data Types

What Are Data Types?

Learn to hack — start here