falsiness

Falsiness is a term used mainly in programming to describe the quality of a value that is interpreted as false in a boolean context. In many languages there is a related notion called truthiness, describing values that evaluate to true. Falsiness results from type coercion and the semantics of boolean expressions, and it is distinct from the philosophical notion of falsity, which refers to a false proposition.

Language-specific rules determine which values are falsy. In JavaScript, for example, false, 0, -0, 0n, '', null, undefined, and NaN are falsy. In Python, the values False and None, the number 0 and 0.0, empty strings and containers ('', [], {}, set(), etc.) are falsy, while other values are truthy. Ruby treats only false and nil as falsy; everything else is truthy. PHP considers false, 0, 0.0, '', '0', null, and empty arrays as falsy, with non-empty strings being truthy.

Practical effects include short-circuit evaluation and the risk of subtle bugs from implicit coercion. Programmers often write explicit checks or use strict comparisons to avoid unintended truthiness, and many languages offer means to coerce values deliberately (for example, Boolean(value) or equivalent). Understanding falsiness helps in writing robust conditionals and in debugging.

Related concepts include truthiness, boolean types, and falsity.