The boolean type has only two values: true and false. Booleans are essential for control flow — every condition in if, while, and loops ultimately evaluates to a boolean (or a value treated as one).

Boolean Literals 

  let isActive = true;
let isDeleted = false;

console.log(typeof isActive); // 'boolean'

Comparison Results

Comparison operators always return booleans:

  console.log(5 > 3);        // true
console.log(5 === '5');    // false
console.log('a' === 'a');  // true

Truthy and Falsy Values

In JavaScript, every value is either truthy or falsy when used in a boolean context.

Falsy values (only 8)

Value Example
false if (false)
0 if (0)
-0
0n BigInt zero
"" Empty string
null
undefined
NaN

Truthy values

Everything else is truthy, including:

  console.log(Boolean('hello'));  // true
console.log(Boolean([]));       // true (empty array is truthy!)
console.log(Boolean({}));       // true
console.log(Boolean('0'));      // true (non-empty string)
console.log(Boolean('false'));  // true

Using Truthy/Falsy in Code 

  let username = '';

// Short-circuit: use default if username is falsy
let displayName = username || 'Guest';
console.log(displayName); // 'Guest'

// Nullish coalescing: only null/undefined trigger default
let count = 0;
console.log(count || 10);  // 10 (0 is falsy)
console.log(count ?? 10);  // 0  (0 is not nullish)

Converting to Boolean 

  Boolean(1);       // true
Boolean(0);       // false
!!'hello';        // true (double negation trick)
!!'';             // false

Practical Examples 

  // Check if array has items
let items = [];
if (items.length) {
    console.log('Has items');
} else {
    console.log('Empty'); // runs
}

// Guard clause
function greet(name) {
    if (!name) {
        console.log('Please provide a name');
        return;
    }
    console.log('Hello, ' + name);
}

greet('');     // Please provide a name
greet('Alice'); // Hello, Alice

Boolean in Logical Expressions 

  // AND returns first falsy or last value
console.log(true && 'yes');   // 'yes'
console.log(false && 'yes');  // false

// OR returns first truthy or last value
console.log(null || 'default'); // 'default'
console.log('hi' || 'default'); // 'hi'

Understanding truthy/falsy behavior helps you write concise conditions and avoid subtle bugs with values like 0 and ''.