Nonextensible

Nonextensible describes a property of an entity that cannot be extended or expanded beyond its current state. The term is used in various domains, but most commonly in computer science to describe objects that cannot gain new properties.

In JavaScript, an object is nonextensible if new properties cannot be added. An object becomes nonextensible by calling Object.preventExtensions(obj). Similarly, Object.seal makes the object nonextensible and also makes all existing properties non-configurable, while Object.freeze makes it nonextensible and all properties non-writable. The function Object.isExtensible(obj) can be used to test whether an object is extensible. Attempts to add properties to a nonextensible object will be ignored or raise a TypeError in strict mode.

In object-oriented programming, nonextensible may describe a class that cannot be subclassed, commonly implemented by using a final specifier (for example, final in Java, sealed in C#, or similar constructs). This ensures the class's behavior cannot be augmented through inheritance. Nonextensibility can also refer more broadly to APIs that prohibit extending their interfaces or behaviors.

In mathematics and formal theories, nonextensible can describe objects or structures that cannot be extended beyond a given domain or constraints, though this usage is less common and context-dependent.