Truthy & Falsy

Truthy and Falsy Values in JavaScript

In JavaScript, every value has an inherent boolean meaning when used in a boolean context. Values that evaluate to true are called truthy, and those that evaluate to false are called falsy.

There are exactly six falsy values: false, 0, "" (empty string), null, undefined, and NaN. Everything else is truthy.

console.log(Boolean("hello"));  // true
console.log(Boolean(42));       // true
console.log(Boolean(0));        // false
console.log(Boolean(""));       // false
console.log(Boolean(null));     // false
console.log(Boolean(undefined)); // false

The double negation !! is a quick way to convert any value to its boolean equivalent. It works exactly like Boolean().

console.log(!!"text");    // true
console.log(!!0);         // false
console.log(!![]);        // true (empty array is truthy)
console.log(!!{});        // true (empty object is truthy)

Truthy and falsy values become especially important in conditionals. Instead of comparing explicitly, you can rely on the truthiness of a value to simplify your code.