JavaScript: Prevent Adding Property

What is Extensible?

• Objects have internal slot “IsExtensible” that indicates if properties can be added to the object.

• the Property Attribute configurable determines if a property can be deleted.
• the Property Attribute writable determines if a property value can be changed.

together, they determine if property can be added, deleted, or value changed.

Warning: parent object may be extensible

Note: if a object is not extensible, but its parent may be, so people can add properties to the parent object, and your object may still get unexpected properties, because of inheritance.

What Objects Are Extensible?

• User-defined objects are extensible, by default.
• Standard objects (For example, Object, Array, Function, Date, etc) are extensible, by default.
• Host objects (for example, windows.document) may or may not be extensible.

console.log(
  [Object, Array, Function, String, Date, RegExp].every((x) =>
    Object.isExtensible(x)
  ),
);

[see Object Type]