Truthy and Falsy Values

Truthy and Falsy Values

What it means

Any JavaScript value can be evaluated as true or false in a boolean context (like an if condition), even if it isn’t actually a boolean. Only a specific, fixed set of values are falsy: false, 0, -0, "", null, undefined, and NaN. Every other value — including "0", [], and {} — is truthy.

Examples

if ("") console.log("truthy");      // doesn't run -- "" is falsy
if ("0") console.log("truthy");     // runs -- non-empty string is truthy!
if (0) console.log("truthy");       // doesn't run -- 0 is falsy
if ([]) console.log("truthy");      // runs -- empty array is truthy!
if ({}) console.log("truthy");      // runs -- empty object is truthy!

const name = "";
console.log(name || "Guest");       // "Guest" -- falls back because "" is falsy
console.log(Boolean(name));         // false -- explicit conversion

console.log(!!"hello");             // true -- double-negation converts to boolean
console.log(!!0);                   // false

Common mistake

Assuming an empty array or object is falsy — both [] and {} are truthy in JavaScript, unlike some other languages, so if (myArray) doesn’t tell you whether the array has items.

const items = [];

if (items) {
  console.log("has items");   // runs! [] is truthy, regardless of length
}

// Correct way to check for an empty array
if (items.length > 0) {
  console.log("has items");   // correctly does NOT run
}

Quick practice

1. List the seven falsy values in JavaScript.

   Answer

   false, 0, -0, "", null, undefined, and NaN.

2. Is an empty array [] truthy or falsy?

   Answer

   Truthy — arrays and objects are always truthy in JavaScript, even when empty.

3. What does !!value do?

   Answer

   Converts value to its explicit boolean equivalent — the first ! negates and coerces to boolean, the second ! negates it back to the "real" truthy/falsy value.