Booleans in JavaScript

Introduction

Booleans (true and false) express yes/no decisions—login allowed, feature enabled, form valid. They power if statements, loops, and logical expressions. This chapter shows how booleans are created, combined, and used without hidden coercion traps.

Prerequisites

• Data types
• Operators overview (logical operators in detail)

Boolean Literals

// Direct true/false values
const isLoggedIn = true;
const hasError = false;
 
console.log(isLoggedIn, hasError);

Booleans from Comparisons

Comparisons produce boolean results:

// Comparison expressions
const age = 20;
const canVote = age >= 18;
const isTeen = age >= 13 && age <= 19;
 
console.log(canVote, isTeen);

Use === and !== for equality (covered in operators).

Truthy and Falsy Values

In conditions, non-boolean values coerce to boolean:

Falsy values:

• false, 0, -0, 0n, "", null, undefined, NaN

Everything else is truthy (including "0" and "false" strings).

// Falsy and truthy in if
const name = "";
 
if (name) {
  console.log("Has name");
} else {
  console.log("Name missing");
}

Logical Operators

Combine boolean expressions:

// Logical combinations
const hasTicket = true;
const age = 17;
const allowed = hasTicket && age >= 18;
 
console.log(allowed);
 
if (!allowed) {
  console.log("Entry denied");
}

Short-circuit behavior:

// && stops at first falsy; || stops at first truthy
const user = null;
const displayName = user && user.name;
console.log(displayName);

Modern alternative for defaults: ?? in operators.

Boolean Conversion

// Explicit Boolean()
const count = 0;
console.log(Boolean(count));
console.log(!!count);

!!value is a common idiom to force true/false.

Mini Example: Form Validation Flags

// Simple validation flags before submit
const email = "user@example.com";
const password = "short";
 
const emailOk = email.includes("@");
const passwordOk = password.length >= 8;
const formValid = emailOk && passwordOk;
 
console.log({ emailOk, passwordOk, formValid });

FAQ

Should I use == or ===?

Prefer === almost always. == performs type coercion and causes subtle bugs.

Is if (flag === true) redundant?

Usually yes—if (flag) is idiomatic when flag is already boolean.

Why is "false" truthy?

It is a non-empty string. Only empty string "" is falsy among strings.

What comes next?

Null and undefined, then type conversion.