JS Booleans

JavaScript Booleans: Truthy, Falsy, and Logical Checks

A JavaScript boolean represents one of two logical states: true or false. Conditional statements like if, while, and the ternary operator evaluate any value as either truthy or falsy.

The Complete List of 8 Falsy Values

Every value in JavaScript is truthy except these eight falsy values:

1. false (boolean false)
2. 0 and -0 (zero)
3. 0n (BigInt zero)
4. "" (empty string)
5. null
6. undefined
7. NaN
8. document.all (legacy browser object)

Notice that empty arrays [], empty objects {}, and the string "false" are all truthy!

Practical Boolean Checking Example

Use Boolean(value) or !!value to coerce values explicitly, and check .length > 0 for arrays instead of if (arr).

const inputs = [0, "", "0", [], {}, null, "active"];

for (const item of inputs) {
  console.log(JSON.stringify(item), "-> Boolean:", Boolean(item));
}

// Explicit checks avoid surprises with 0 or []
const itemCount = 0;
if (itemCount !== null && itemCount !== undefined) {
  console.log("Valid numeric count:", itemCount);
}

Common Mistakes to Avoid

• Writing if (items) to check if an array is non-empty: An empty array [] is truthy! Always write if (items.length > 0).
• Using if (userInput) when 0 is valid: If a user enters 0 for quantity, if (qty) treats 0 as falsy and skips it. Check qty !== undefined or typeof qty === "number".
• Creating new Boolean(false): Wrapping a boolean in new Boolean(false) creates an object, and every object is truthy inside if (new Boolean(false))!

Frequently Asked Questions

Why is "0" truthy while 0 is falsy?

"0" is a non-empty string with length 1. Only the empty string "" is falsy among strings.

What does the double negation !!value do?

The first ! flips the value's truthiness to a boolean, and the second ! flips it back, converting any expression into a strict true or false. It works identically to Boolean(value).

How do I check if a plain object has no properties?

Use Object.keys(obj).length === 0, because {} itself is truthy.