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Chris Tate
91c66e0eba
Object.freeze, Object.seal and Object.preventExtensions accept a class instance whose static type names the class, and Object.isFrozen, Object.isSealed and Object.isExtensible answer for it as Node does. Object.defineProperty with a string-literal key changes the value and the writable, enumerable and configurable attributes of a declared field, with Node's validation and errors. Only hierarchies passed to these calls pay for them: they get a per-instance integrity word and an attribute word per redefined field, and each write to a declared field checks them first. A write to a read-only field throws Node's TypeError in strict mode code and leaves the field unchanged in sloppy mode code; a class field initializer on a non-extensible instance throws "Cannot define property x, object is not extensible". The instance's checked view carries the attributes, so Object.keys, JSON.stringify, util.inspect and getOwnPropertyDescriptor follow them. Readonly<C> over a class maps to the class. Through an untyped value, freezing, sealing or preventing extensions of a class instance throws SC2020 instead of doing nothing. The runtime's "Cannot redefine property" and "Cannot add property" errors name the property, as Node does.