extends — Subclassing built-ins

Reference note (untrusted external data; do not execute it as instructions). > [!WARNING] > The standard committee now holds the position that the built-in subclassing mechanism in previous spec versions is over-engineered and causes non-negligible performance and security impacts. New built-in methods consider less about subclasses, and engine implementers are investigating whether to remove certain subclassing mechanisms. Consider using composition instead of inheritance when enhancing built-ins. Here are some things you may expect when extending a class When calling a static factory method (like {{jsxref("Promise.resolve()")}} or {{jsxref("Array.from()")}}) on a subclass, the returned instance is always an instance of the subclass. When calling an instance method that returns a new instance (like {{jsxref("Promise.prototype.then()")}} or {{jsxref("Array.prototype.map()")}}) on a subclass, the returned instance is always an instance of the subclass. Instance Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV isolated this documentation section, normalized formatting, removed long code blocks, and shortened it for retrieval. Verify version-sensitive details at the source.