Start with the idea

A boolean is true or false. Use comparisons to give business rules a clear name, then use that boolean in a decision.

Step-by-step lesson notes

Booleans in JavaScript

A boolean represents one of two values: true or false. Booleans are used in conditions, comparisons, and logical operations.

Boolean Values

let isLoggedIn = true;
let hasErrors = false;

// Boolean() converts any value to true/false
Boolean(1);          // true
Boolean(0);          // false
Boolean("hello");    // true
Boolean("");         // false
Boolean(null);       // false
Boolean(undefined);  // false
Boolean([]);         // true  ← empty array is truthy!
Boolean({});         // true  ← empty object is truthy!

Falsy and Truthy Values

Every value in JavaScript is either truthy or falsy when used in a boolean context (like an if condition).

// Falsy in if conditions
if (0)         { /* won't run */ }
if ("")        { /* won't run */ }
if (null)      { /* won't run */ }
if (undefined) { /* won't run */ }

// Truthy in if conditions
if ("0")  { /* WILL run — non-empty string */ }
if ([])   { /* WILL run — empty array is truthy! */ }
if ({})   { /* WILL run — empty object is truthy! */ }

// Double negation trick — convert to boolean
const hasValue = !!someVariable;
console.log(!!"hello"); // true
console.log(!!"");      // false
console.log(!!0);       // false
console.log(!!null);    // false

A complete example

Run this complete index.js program in the JavaScript console editor below. The earlier reference fragments may need a browser page or other surrounding setup.

const age = 18;
const hasTicket = true;
const canEnter = age >= 18 && hasTicket;
console.log(canEnter);
console.log(Boolean("false"));

true
true

Make one change. See what happens.

Load the example, change a value, predict the output, and run it again. Use the console output to check your reasoning.

Wait for the editor to load, then load this example and press Run.

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Put the idea to work

Your learning checkpoint

• Explain the example in your own words.
• Complete the challenge before revealing its answer.
• Describe the common mistake and how to avoid it.