Chapter 03

Data Types

Not all values are the same kind of thing. The number 42 behaves differently from the word "hello", which behaves differently from a yes/no answer. These different kinds are called types. JavaScript has a handful of basic ones, and you'll use three of them constantly: numbers, strings, and booleans.

Numbers

Numbers are written just as you'd expect - no quotes. Whole numbers and decimals are both just "numbers" in JavaScript; there's no separate type for them.

const apples = 5;
const price = 2.50;
const temperature = -3;

console.log(apples + 2);  // 7
console.log(price * 4);   // 10

You can do arithmetic directly: + adds, - subtracts, * multiplies, / divides. We'll cover all of these in the next chapter.

Strings (Text)

A string is a piece of text. It's called a "string" because it's a string of characters joined together. You must wrap it in quotes - single ' ' or double " ", it doesn't matter, as long as they match:

const greeting = "Hello";
const name = 'Ada';
console.log(greeting); // Hello

The quotes are not part of the value - they just tell JavaScript "this is text." Without them, JavaScript thinks you're referring to a variable and gets confused:

const city = London; // Error! London is not defined.
const city2 = "London"; // Correct - it's text.

Note

A number in quotes is a string, not a number: "5" is text, 5 is a number. They look similar but behave very differently, which trips up almost every beginner. We'll see why in Chapter 5.

Booleans (True or False)

A boolean has only two possible values: true or false. That's it. They answer yes/no questions and are the foundation of decision-making (Chapter 6):

const isRaining = true;
const hasFinished = false;
console.log(isRaining); // true

Note there are no quotes - true is a special built-in value, whereas "true" would just be the word "true" as text.

Nothing: null and undefined

Two special values represent "no value." They're subtly different:

undefined means "this hasn't been given a value yet." A variable you declare but don't assign is undefined.
null means "deliberately empty." You use it to say "there is nothing here on purpose."

let winner;
console.log(winner); // undefined - never assigned

let selected = null;
console.log(selected); // null - intentionally empty

You'll meet these constantly, usually when something you expected to be there isn't. For now, just recognise them.

Checking a Value's Type

If you're ever unsure what type a value is, ask JavaScript with typeof:

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

Tip

When a program misbehaves, a typeof check is often the fastest way to spot the problem - very often a value you thought was a number is secretly a string.

Chapter 2: Values & Variables Chapter 4: Operators & Expressions