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Truthy & Falsy Values

Truthy & Falsy Values

Introduction

In JavaScript, every value has an inherent boolean “truthiness” — it’s either truthy (evaluates to true in a boolean context) or falsy (evaluates to false).

Why Do We Need This?

Truthy/falsy checks are used everywhere: if statements, logical operators &&/||, and ternary expressions. Understanding them is key to writing clean conditional logic.

Falsy Values

There are exactly 6 falsy values in JavaScript:

false;        // the boolean false
0;            // zero
'';           // empty string
null;         // null
undefined;    // undefined
NaN;          // Not a Number

Truthy Values

Everything else is truthy:

true;
1;              // any non-zero number
'hello';        // any non-empty string
[];             // empty array
{};             // empty object
function(){};   // any function
'false';        // the string "false" — truthy!

Visual Explanation

flowchart TD
    Value[Is this value falsy?]
    Check{"Value === false / 0 / '' / null / undefined / NaN?"}
    Yes["⬇️ Evaluates to false"]
    No["⬇️ Evaluates to true"]

    Value --> Check
    Check -->|Yes| Yes
    Check -->|No| No

Practical Usage

// Default values with ||
const name = userInput || 'Guest';
// If userInput is falsy (empty, null, undefined), use 'Guest'

// Guard with &&
isLoggedIn && renderDashboard();
// Only render if isLoggedIn is truthy

Common Mistakes

// ❌ Wrong
if (value == false) { } // Too broad — catches 0 and ''

// ✅ Right
if (value === false) { } // Only actual false

// ❌ Wrong — 0 is falsy
if (score) { } // Fails when score is 0

// ✅ Right
if (score !== null && score !== undefined) { }

Summary

Only 6 falsy values: false, 0, '', null, undefined, NaN
Everything else is truthy
Use !! to check truthiness: !!value
Be careful with 0 — it’s falsy but often a valid value

Further Reading