orundefined

Orundefined is a term used in type systems to describe a value that may be either a specific type or undefined. It is a way to model optional data, where the absence of a value is represented by the special value undefined rather than a separate null value.

In TypeScript and related typing ecosystems, orundefined is typically expressed as a union with undefined. The standard form is T | undefined, meaning a value is either of type T or not provided at all. Some codebases adopt a named alias for readability, such as type OrUndefined<T> = T | undefined. This alias is not built into the language by default but is provided by libraries like type-fest and can be defined in user code.

Usage examples include function return types, variables, and object properties that may be omitted. For instance, a function might return string | undefined if a value might be missing, and an object property declared as property?: string has the effective type string | undefined. When using orundefined, developers often perform type narrowing to handle the undefined case, or supply default values to coerce to a defined type.

Orundefined is distinct from null in languages where both concepts exist; undefined typically signals the absence of a value rather than an explicit null result. Understanding orundefined helps in writing robust code that explicitly accounts for optional data and reduces runtime errors related to undefined values.

See also: Optional types, Union types, Undefined, Null, TypeScript type aliases.