Extension
Also called: Extension function, Extension method
A way to add methods or computed properties to an existing type, even one you didn't write, without changing or subclassing it.
Definition
Swift extensions and Kotlin extension functions let you call new helpers with normal dot syntax, such as name.initials. They can't store new data or override existing methods, and they are chosen by the variable's declared type at compile time. Java has no extensions and uses static helper methods instead; TypeScript uses plain functions.
In practice
extension Date { var isWeekend: Bool { … } } lets any Swift code write date.isWeekend.