Values and Data Types

Values and Data Types

In this chapter, you will learn what a value is and what a data type is. A value is a single piece of data, like the number 7 or the word "hello". A data type is the kind of value it is. JavaScript uses the type to decide what it can do with a value — for example, you can do math with numbers, but not with words.

Why Types Matter

Every value in JavaScript has exactly one type. When you combine values, JavaScript follows rules based on those types. If you know the type, you can predict what your code will do. If you don't, you get surprises — like "2" + 2 turning into the string "22" instead of the number 4.

The Core Types

Strings

A string is text wrapped in quotes. It can hold letters, numbers, spaces — anything you can type.

const greeting = "Hello, world!";
const name = 'Ada';

You can use double quotes or single quotes. Backticks (` ``) also make strings and let you insert values inside them:

const name = 'Ada';
const message = `Hi, ${name}!`; // "Hi, Ada!"

Numbers

A number is a value for math. JavaScript has one number type for both whole numbers and decimals.

const age = 30;      // whole number
const price = 9.99;  // decimal

Booleans

A boolean is a value that is either true or false. Booleans power decisions, like "is the user logged in?"

const isLoggedIn = true;
const hasPermission = false;

null and undefined

These two types both mean "no value," but in different ways:

• null means "intentionally empty." A programmer set it on purpose.
• undefined means "not set yet." If you create a variable and never give it a value, it is undefined.

let nextStep;          // undefined — nothing assigned yet
nextStep = null;       // null — deliberately empty for now

Checking a Type with typeof

The typeof operator tells you the type of any value as a string:

console.log(typeof "hi");      // "string"
console.log(typeof 42);        // "number"
console.log(typeof true);      // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null);      // "object" (a famous old bug!)

Putting Types Together

Try this in your browser console (press F12 and click the Console tab):

console.log(2 + 2);        // 4 — number plus number
console.log("2" + 2);      // "22" — string plus number makes a string!
console.log("Hi " + 5);    // "Hi5"
console.log(10 > 3);       // true — comparison gives a boolean

Notice "2" + 2: when you use + with a string, JavaScript converts the other value to a string and glues them together. With any other math operator, it tries to convert to a number instead:

console.log("6" - 2);  // 4
console.log("6" * 2);  // 12

The safe habit: keep your types consistent, and use typeof when you are unsure.