Stage 0 Draft / April 17, 2025

Disallow Adding New Private Fields to Non-extensible Objects

1 Modifications to Operations on Objects

1.1 PrivateFieldAdd ( O, P, value )

The abstract operation PrivateFieldAdd takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If O.[[Extensible]] is false, throw a TypeError exception.
  2. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  3. Let entry be PrivateElementFind(O, P).
  4. If entry is not empty, throw a TypeError exception.
  5. Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
  6. Return unused.

1.2 PrivateMethodOrAccessorAdd ( O, method )

The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and method (a PrivateElement) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: method.[[Kind]] is either method or accessor.
  2. If O.[[Extensible]] is false, throw a TypeError exception.
  3. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  4. Let entry be PrivateElementFind(O, method.[[Key]]).
  5. If entry is not empty, throw a TypeError exception.
  6. Append method to O.[[PrivateElements]].
  7. Return unused.

A Copyright & Software License

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© 2025 Shu-yu Guo

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