JS◆ The JavaScript Book
About

Chapter IV · Operators & Expressions

Truthy and Falsy Values

Every value in JavaScript is either "truthy" or "falsy" when used where a boolean is expected — and there are exactly eight falsy values, everything else is truthy.

01 What Are Truthy and Falsy Values?

Conditions in if, while, and logical operators don't require an actual boolean — they coerce whatever they're given into one. This exists so you can write if (user) instead of if (user !== null && user !== undefined).

The falsy values are a short, memorizable list: false, 0, -0, 0n, '', null, undefined, NaN. Everything else — including '0' the string, empty arrays [], and empty objects {} — is truthy.

02 Simple Example

The full falsy list, and a couple of common surprises.

truthy-falsy.js
if (0) console.log('never runs');
if ('0') console.log('this runs — non-empty string is truthy');
if ([]) console.log('this runs — even an empty array is truthy');

03 How It Works

When a value appears where a boolean is required, the engine runs it through the abstract ToBoolean operation from the spec — a lookup, not a calculation. It checks whether the value matches one of the eight falsy cases; if not, the result is true.

04 Production Example

Guarding against missing or empty data before rendering it.

render-cart.js
function renderCartSummary(items) {
  if (!items || items.length === 0) {
    return 'Your cart is empty.';
  }
  return `${items.length} item(s) in your cart.`;
}

05 Mental Model

"A short blocklist, not a checklist: JavaScript only treats eight specific values as 'off' — everything else defaults to 'on.'"

06 Common Mistakes

  • Assuming empty arrays/objects are falsy — they aren't; check .length or Object.keys().length explicitly.
  • Using if (count) to check "has a value" when 0 is a valid, meaningful count — that condition silently treats 0 as missing.

07 Interview Questions

Q: Name all eight falsy values.

false, 0, -0, 0n, '', null, undefined, and NaN — every other value, including [] and {}, is truthy.

Q: Why is if (count) risky when count could be 0?

It silently treats a real, meaningful 0 the same as missing data — use an explicit check like count !== undefined when 0 is valid.

Key Takeaway

Only eight values are falsy (false, 0, -0, 0n, "", null, undefined, NaN); everything else, including [] and {}, is truthy — worth checking explicitly when 0 or "" are meaningful data.

Related Concepts