JS: Prevent Adding Property

What is Extensible?

• 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.

Parent object may be extensible

🛑 WARNING: 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 (e.g. Object, Array, Function, Date, etc) are extensible, by default.
• Host objects (e.g. windows.document) may or may not be extensible.

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

〔see Object Type〕

Check If Object is Extensible

• JS: Object.isExtensible
• JS: Reflect.isExtensible

Prevent Adding Properties

• JS: Object.preventExtensions

Prevent Adding/Deleting Properties

• JS: Object.seal
• JS: Object.isSealed

Prevent Adding/Deleting/Writing Properties

• JS: Object.freeze
• JS: Object.isFrozen