nonenumerable

Nonenumerable describes a property that is not included when properties are enumerated. In many programming contexts, enumeration refers to iterating over an object's properties, such as with loops or methods that list keys. A nonenumerable property is one that does not appear in such enumerations, although it can still be accessed directly by name.

In JavaScript, property enumeration is controlled by the enumerable attribute of a property descriptor. By default, properties created with simple assignment are enumerable, while others can be defined as non-enumerable using Object.defineProperty or Object.defineProperties with enumerable set to false. For example, defining a non-enumerable property means it will not show up in for...in loops or Object.keys, and it will be skipped by JSON.stringify. Nevertheless, the value remains accessible via obj.prop or obj['prop'] if you know the name, and it can be listed by methods that expose non-enumerable properties, such as Object.getOwnPropertyNames, which returns all own properties regardless of enumerability, or Object.getOwnPropertyDescriptors.

Non-enumerable properties are commonly used to hide internal state, metadata, or implementation details from typical object enumerations, while leaving them accessible for programmatic access. They are not a security feature; determined code can still read them. An alternative approach for true non-visibility is to use Symbol-keyed properties or private class fields (introduced in modern JavaScript), which offer different semantics for hiding properties from ordinary enumeration.

In summary, nonenumerable is a property characteristic that affects visibility during enumeration, not direct accessibility.