Variables and Data Types

Introduction

Programs need to remember things: a player's score, a name, whether someone is logged in. A variable is how a program remembers.

The simple explanation

A variable is a labelled box that holds a value.

You give the box a name, put a value inside, and use the name whenever you need the value again.

• let creates a variable.
• age is its name.
• = means store the value on the right into the name on the left. It is not "equals" like in maths.

let and const

• let is for values that will change.
• const is for values that will not be reassigned. Prefer it by default.

The last line throws an error: you cannot reassign a const. You may also see var in old code. Avoid it in new code.

Naming variables

• Start with a letter, _ or $. Never start with a number.
• Names are case-sensitive: score and Score are different.
• The usual style is camelCase: firstName, totalPrice.
• Choose meaningful names: price is better than p.

Data types

Values come in different types. The basic ones:

• String: text, in quotes: "hello" or 'hello'.
• Number: whole or decimal numbers: 42, 3.14.
• Boolean: true or false.
• undefined: a variable that has no value yet.
• null: an intentional "nothing".

typeof tells you the type of a value.

The last line prints object. That is a famous historical quirk of JavaScript. null is not really an object.

Template literals

Backticks let you put variables inside text using ${ }.

Try it yourself

Make variables for your name, your favourite number and whether you like pizza (a boolean). Print one sentence using all three with a template literal.

Summary

• Variables store values: let for changing values, const for fixed ones.
• Basic types: string, number, boolean, undefined and null.
• typeof shows a value's type.
• Template literals mix variables into text: ${ }.