Boolean

JavaScript · Reference cheat sheet

📋 Overview

Booleans are true / false. Many values coerce via ToBoolean in conditionals. Prefer explicit checks over truthiness when 0, "", or null are valid data.

🔧 Core concepts

Literals: true, false.
Boolean(x) / !!x: convert with ToBoolean.
Falsy: false, 0, -0, 0n, "", null, undefined, NaN.
Truthy: everything else (including [], \{\}, "0").
Logical ops: &&, ||, ?? (nullish), !.
Comparisons: === / !== preferred over == / !=.

Boolean(0); // false
Boolean([]); // true
!!"hi"; // true

💡 Examples

// Nullish vs OR
const port = settings.port ?? 3000; // only null/undefined
const title = name || "Guest"; // also replaces ""

// Short-circuit
user && user.profile && user.profile.id;
user?.profile?.id; // optional chaining

// Guards
function assert(cond, msg) {
  if (!cond) throw new Error(msg);
}

// Explicit empty checks
function hasText(s) {
  return typeof s === "string" && s.trim().length > 0;
}

// Toggle
let on = false;
on = !on;

// Boolean object trap
const boxed = new Boolean(false);
if (boxed) {
  // runs — object is truthy!
}

// Filter truthy
const values = [0, 1, "", "a", null, undefined, NaN, false, true];
const truthy = values.filter(Boolean); // [1, "a", true]

// Predicate helpers
const isNil = (v) => v == null; // null or undefined
const isPresent = (v) => v != null;

⚠️ Pitfalls

new Boolean(false) is an object and is truthy — never use Boolean wrappers.
== coerces: false == 0, "" == 0 — use ===.
Empty arrays/objects are truthy — check .length or Object.keys.
?? only skips null/undefined; || also skips other falsy values.
Document APIs that return 0 or "" so callers do not treat them as missing.

number.md — numeric falsy values
strings.md — empty strings
objects.md — nullish patterns
switch_case.md — branching
error.md — assertions

Boolean

Boolean

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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