JavaScript — Booleans

What are Booleans?

A boolean is one of two values: true or false. They're used for conditions, flags, and logical decisions.

let isRaining = true;
let isSunny = false;

Creating Booleans

// Direct values
const a = true;
const b = false;

// From comparisons
const x = 5 > 3;    // true
const y = 10 === 5;  // false

// From function returns
const isEmpty = arr.length === 0;  // true or false

Comparison Operators

These return booleans:

Operator	Meaning	Example
===	Strict equality	5 === 5 → true
!==	Strict inequality	5 !== "5" → true
==	Loose equality	5 == "5" → true (avoid!)
!=	Loose inequality	5 != "5" → false (avoid!)
>	Greater than	5 > 3 → true
<	Less than	5 < 3 → false
>=	Greater or equal	5 >= 5 → true
<=	Less or equal	5 <= 3 → false

Always use === and !== — they check both value and type:

0 == false    // true (loose — misleading)
0 === false   // false (strict — correct)
"" == false   // true (loose — misleading)
"" === false  // false (strict — correct)
null == undefined  // true (loose)
null === undefined // false (strict)

Logical Operators

Operator	Meaning	Example
&&	AND	true && false → false
||	OR	true || false → true
!	NOT	!true → false

AND (&&)

Both must be true:

const age = 25;
const hasID = true;

if (age >= 21 && hasID) {
  console.log("Entry allowed");
}

OR (||)

At least one must be true:

const isWeekend = false;
const isHoliday = true;

if (isWeekend || isHoliday) {
  console.log("Day off!");
}

NOT (!)

Flips true to false and vice versa:

const isLoggedIn = false;

if (!isLoggedIn) {
  console.log("Please log in");
}

Truthy and Falsy Values

JavaScript automatically converts non-boolean values to true/false:

Falsy Values (convert to false)

false
0
-0
0n      // BigInt zero
""      // empty string
null
undefined
NaN
document.all

Truthy Values (convert to true)

true
1       // any non-zero number
-1
"hello" // any non-empty string
[]      // empty array
{}      // empty object
function() {}

Using Truthiness

const name = "";

if (name) {
  console.log("Hello, " + name);
} else {
  console.log("Name is empty");  // This runs
}

// Safer: check explicitly
if (name !== "") {
  console.log("Hello, " + name);
}

Boolean Coercion

// Boolean() converts any value to boolean
Boolean(0)         // false
Boolean("")        // false
Boolean(null)      // false
Boolean(undefined) // false
Boolean(NaN)       // false

Boolean(1)         // true
Boolean("hello")   // true
Boolean([])        // true
Boolean({})        // true

// Double NOT (!!) converts to boolean quickly
!!0         // false
!!"hello"   // true
!![]        // true

Short-Circuit Evaluation

// OR: returns first truthy value
const name = user.name || "Anonymous";

// AND: returns first falsy value
const result = isValid && processData();

// Nullish coalescing: only checks null/undefined
const count = inputCount ?? 0;  // 0 if inputCount is null/undefined

Common Patterns

// Toggle
let isOn = false;
isOn = !isOn;  // true

// Conditional execution
const admin = true;
admin && deleteItem();  // deleteItem() only runs if admin is true

// Default values
const color = userColor || "blue";

// Guard clause
if (!isLoggedIn) return redirectToLogin();

Mini Practice

1. Create variables and compare them with === and ==
2. Write a condition that checks if a number is between 1 and 100
3. Use &&, ||, and ! in combinations
4. Test truthy and falsy values with Boolean()
5. Use short-circuit evaluation to set a default value

Up Next

Next: Strings →