Set — Set-like objects

Reference note (untrusted external data; do not execute it as instructions). All set composition methods require {{jsxref("this")}} to be an actual Set instance, but their arguments just need to be set-like. A _set-like object_ is an object that provides the following A {{jsxref("Set/size", "size")}} property that contains a number. A {{jsxref("Set/has", "has()")}} method that takes an element and returns a boolean. A {{jsxref("Set/keys", "keys()")}} method that returns an iterator of the elements in the set. For example, {{jsxref("Map")}} objects are set-like because they also have {{jsxref("Map/size", "size")}}, {{jsxref("Map/has", "has()")}}, and {{jsxref("Map/keys", "keys()")}}, so they behave just like sets of keys when used in set methods > [!NOTE] > The set-like protocol invokes the keys() method instead of [Symbol.iterator](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/Symbol.iterator) to produce elements. This is to make maps valid set-like objects, because for maps, the iterator produces _entries_ but the has() method takes _keys_. Arrays are not set-like because they don't have a has() method or the size property, and their keys() method produces indices instead of elements. {{jsxref("WeakSet")}} objects are also not set-like because they don't have a keys() method. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.