Conversion de types : modèles sûrs pour éviter les surprises

Introduction

Démonstration de conversion implicite

// Implicit coercion can surprise beginners
console.log("5" + 1);     
// "51"  (string concatenation)
console.log(5 + "1");    
// "51"  (number -> string)
console.log("5" - 1);     
// 4     (string -> number)
console.log("5" * 2);     
// 10    (string -> number)

Conversions explicites

// Prefer explicit, readable casts
const n = Number("42");      
// 42
const s = String(42);         
// "42"
const t = Boolean("hi");      
// true (non-empty string)
const f = Boolean("");        
// false (empty string)

console.log(n, s, t, f);

Number et parseInt

// Number vs parseInt with examples
console.log(Number("42"));          
// 42 (full string must be numeric)
console.log(parseInt("42", 10));    
// 42
console.log(parseInt("42px", 10));  
// 42 (stops at non-digit)
console.log(Number("42px"));        
// NaN (not a pure number)

Vérifications de NaN

// NaN is "Not a Number" and is not equal to itself
const bad = Number("oops");   
// NaN
console.log(bad === NaN);     
// false
console.log(Number.isNaN(bad)); 
// true (use this)
console.log(isNaN("5"));      
// false (string coerces to number 5)

Liste de vérification de sécurité

Question sur la conversion numérique explicite

Récapitulatif