Nullundefined

Nullundefined is a term used in programming discussions to describe the relationship between null and undefined, two ways languages express the absence of a value. The phrase highlights that, while related, null and undefined are not interchangeable and combining them can lead to ambiguity, especially in loosely typed languages where values can flow through without explicit checks.

In JavaScript, null is a value of the object type that explicitly indicates "no value here," while undefined indicates that a variable has been declared but not assigned, or that a property is missing. They are distinct types, yet the loose equality operator (==) treats them as equal, whereas strict equality (===) does not. The typeof operator yields "object" for null and "undefined" for undefined, which can contribute to confusion. Modern JavaScript provides the nullish coalescing operator (??) and optional chaining (?.) to help handle both null and undefined in a unified way.

Languages vary in their treatment. Some ecosystems distinguish null and undefined more clearly, others provide a single absence value. In SQL, NULL represents unknown or inapplicable data, and comparisons involving NULL generally yield unknown rather than true or false. Type systems may also offer optional types to distinguish the two, aiding compile-time safety.

Practically, developers manage null and undefined with explicit checks, defaults, or language features designed to address absence values. Recognizing the difference helps prevent runtime errors and improves code reliability when accessing properties or performing value-based logic.

See also: null, undefined, JavaScript, TypeScript, nullish coalescing, optional chaining.