filterByType

FilterByType is a utility pattern that selects elements from a collection whose runtime type matches a specified type. It helps organize mixed data and enables type-specific processing without introducing ad hoc filtering logic throughout code. The function typically takes a collection and a type descriptor and returns a new collection composed only of items that satisfy the type criterion.

Implementation details vary by language. In dynamically typed languages, the type descriptor is usually a class or constructor, and the check relies on runtime checks such as isinstance, isInstance, or equivalent. In statically typed languages, filterByType may use generics with runtime type checks or pattern matching to filter elements that conform to a given type at runtime.

Common use cases include separating objects in a heterogeneous list into groups by their type, applying different workflows to each subgroup, and safely downcasting elements after filtering. It can also support data validation by collecting elements of a particular type for independent validation paths.

Caveats include ambiguity with inheritance where subclasses also match the type, potential performance costs from repeated type checks, and reliance on runtime type information that may not be available in all contexts. When using type descriptors that are not unique, the precise meaning of a match may vary between languages and libraries.

See also: filter, type checking, type guard, isInstance.