falsey

Falsey is a term used in programming to describe values that evaluate to false in a boolean context. The spelling falsey is common, though many communities also use falsy; both refer to the same concept of values that do not pass a truth test.

The exact set of falsey values is language dependent, but several patterns recur. In JavaScript, falsey values include false, 0, -0, 0n (zero BigInt), "", null, undefined, and NaN. All other values evaluate to true in a conditional sense. In Python, falsey values include False, None, 0, 0.0, 0j, '', (), [], {}, set(), and other empty containers; nonempty or nonzero values are truthy. In Ruby, only false and nil are falsey; everything else is considered truthy. In PHP, falsey values include false, 0, 0.0, '', '0', [], and null, with type juggling causing some surprising cases (for example, the string "0" is considered falsey).

Implications of falsey values appear most directly in conditional statements, where a falsey value will skip the corresponding branch, while a truthy value will execute it. Developers must understand a language’s rules for truthiness to avoid logical errors, especially when performing type coercion or testing user input.

See also: falsy values, truthiness, type coercion.