Variables, Types, and Operators

JavaScript is the programming language of the Web: it runs in every browser to make pages interactive, and on the server with Node.js. In this course, our exercises run with Node, without a web page: we focus on the language itself.

Declaring variables

const prixUnitaire = 19.9;   // const: cannot be reassigned
let quantite = 2;            // let: can change
quantite = 3;                // OK
// prixUnitaire = 25;        // TypeError: assignment to a constant

console.log(prixUnitaire * quantite); // prints 59.7 (approximately, see below)

Simple rule: use const by default, and let only if you must reassign the variable. The old var keyword has tricky behaviors (function scope, declaration “hoisting”): it is no longer used in modern code.

Names follow the camelCase convention (prixTotal, estConnecte) and should be meaningful.

Primitive types

const nom = "Léa";             // string
const age = 31;                // number (integers and decimals)
const estAbonne = true;        // boolean
let commentaire;               // undefined: declared without a value
const coupon = null;           // null: deliberate absence of a value
const grandNombre = 9007199254740993n; // bigint: arbitrarily large integers

console.log(typeof nom);       // "string"
console.log(typeof age);       // "number"
console.log(typeof null);      // "object": a historical quirk to know about

There is also the symbol type, which is rarer. Everything else (arrays, objects, functions) is of the object type.

Operators and strings

const total = 10 + 5 * 2;      // 20: multiplication takes precedence
const reste = 17 % 5;          // 2: remainder of the division (modulo)
const carre = 3 ** 2;          // 9: exponentiation
let compteur = 0;
compteur += 1;                 // compteur = compteur + 1
compteur++;                    // increments by 1

// Template literals (backticks): insert expressions with ${...}
const message = `Hello ${nom}, you are ${age} years old.`;
const ligne = `Total : ${(19.9 * 3).toFixed(2)} €`;  // "Total : 59.70 €"

Strings have many methods: nom.length, nom.toUpperCase(), texte.trim(), texte.includes("@"), texte.split(","), texte.padStart(2, "0").

Comparing: always with ===

5 === 5        // true
5 === "5"      // false: different types
5 == "5"       // true: == converts types before comparing (trap)
0 == false     // true (!)
null == undefined // true

=== and !== compare without conversion: use them systematically. Other operators: <, >, <=, >=, and the logical operators && (and), || (or), ! (not).

Converting explicitly

Data coming from a form or a URL are strings. You need to convert them:

const saisie = "42";
const n = Number(saisie);           // 42
"3" + 4                             // "34": + concatenates if a string is present
Number("3") + 4                     // 7
Number("douze")                     // NaN (Not a Number)
Number.isNaN(Number("douze"))       // true
Number.parseFloat("12.5 kg")        // 12.5: reads the start of the string
String(42)                          // "42"

The decimal trap

0.1 + 0.2              // 0.30000000000000004

Numbers are stored in binary floating point: some decimals cannot be represented exactly. For money, calculate in whole cents (1990 rather than 19.90) and format only for display.