falsy

Falsy is a term used in programming to describe values that evaluate to false when tested in a boolean context. In many languages, conditionals like if statements coerce values to boolean, so a falsy value is interpreted as false and a truthy value as true.

Which values are falsy depends on the language. In JavaScript, common falsy values include false, 0, empty string "", null, undefined, and NaN. In Python, falsy values include False, None, 0 and 0.0, empty sequences or containers such as "", (), [], {}, and set(), and also None. In PHP, falsy results occur for false, 0, 0.0, '', '0', [], null, and similar empty types.

Programs rely on falsy evaluation for control flow, defaulting, and short-circuit logic. However, relying on implicit coercion can introduce bugs, especially in languages with loose equality semantics or ambiguous truthiness rules. To reduce surprises, developers often prefer explicit checks or strict comparisons, or use language features that make truthiness more predictable. For example, in JavaScript, strict equality (===) avoids coercion, while in Python the truthiness of a value is handled by the if statement itself, with more explicit logic available when needed.

Understanding falsy values helps in reasoning about conditional logic, input validation, and defaulting behavior across languages. It highlights how different types and values interact with boolean contexts and why careful handling of truthiness matters in software design.