Variables and data types

A game remembers your name, your score and which level you are on. An online shop knows how many items are in your basket and what they cost in total. Programs remember information in variables — think of them as boxes with a label on them.

Creating a variable: let

We declare a variable with the keyword let and assign it a value with the = sign. Careful: in programming = does not mean “equals” — it means “store the value on the right in the variable on the left”. That is why score = score + 5 is a perfectly valid statement: 5 is added to the old value.

let score = 0;
console.log('Start:', score);

score = 10;
console.log('After level 1:', score);

score = score + 5;
console.log('After bonus:', score);

Start: 0
After level 1: 10
After bonus: 15

We write let only when we create the variable for the first time. For later changes the name and the new value are enough.

Constants: const

Some values never change while a program runs: the name of a country, the number of days in a week, a tax rate. For these we use const (short for constant). A variable created with const cannot be assigned a new value.

const country = 'Azerbaijan';
const capital = 'Baku';
let temperature = 18;

temperature = 21;
console.log(capital, 'is the capital of', country);
console.log('Temperature:', temperature);

Baku is the capital of Azerbaijan
Temperature: 21

const capital = 'Baku';
capital = 'Ganja';

Uncaught TypeError: Assignment to constant variable.

Naming variables

A name may contain letters, digits and the signs _ and $, but it cannot start with a digit or contain spaces. Keywords such as let, const or if cannot be names. In JavaScript, names made of several words are written in camelCase: the first word starts with a small letter, every next word with a capital.

Data types and typeof

Every value has a type. The type decides what you can do with the value: numbers can be added, text can be turned into capital letters. The typeof operator returns a value's type as a string.

console.log(typeof 42);
console.log(typeof 3.14);
console.log(typeof 'Leyla');
console.log(typeof '42');
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof null);

number
number
string
string
boolean
undefined
object

JavaScript is a dynamically typed language: the same variable can hold a number first and a string later — the type belongs to the value, not to the variable. undefined means “nothing has been stored yet”, while null means “there is deliberately nothing here”.

let value = 42;
console.log(value, typeof value);
value = 'forty-two';
console.log(value, typeof value);

let winner;
console.log('Before the game:', winner);
winner = null;
console.log('Draw, no winner:', winner);
winner = 'Murad';
console.log('Winner:', winner);

42 number
forty-two string
Before the game: undefined
Draw, no winner: null
Winner: Murad

let city = 'Baku';
let temperature = 18;
// 1) increase temperature by 5
// 2) print the city and the temperature
// 3) print the types of both variables

Baku 23
string number