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Normative: allow host exotic objects to reject private fields #2807

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Aug 12, 2022
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Normative: allow host exotic objects to reject private fields #2807

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Normative: allow host exotic objects to reject private fields (#2807)
bakkot Aug 12, 2022
4bfe505
Editorial: note browser-specific host hooks in Annex B (#2807)
bakkot Aug 12, 2022
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33 changes: 33 additions & 0 deletions spec.html
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Expand Up @@ -6816,6 +6816,8 @@ <h1>
<dl class="header">
</dl>
<emu-alg>
1. If the host is a web browser, then
1. Perform ? HostEnsureCanAddPrivateElement(_O_).
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This is the first time that I see something like this in an algo step, I have mix feelings about it.

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@phoddie requested that this be Annex B only, so that non-browser hosts wouldn't have to read it at all. But I really do not want to put any more things in Annex B. So this was the compromise.

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While I do think it seems weird, and I think it's far more appropriate to have the check inside the host hook, this is "fine" imo - and I believe this represents pushback from @phoddie (who may want to clarify)

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I must agreed that having this in the algos rather than in a separate sections that nobody looks at is a lot better, I was just hesitant because this is the first time I have seen this condition in the spec.

1. Let _entry_ be PrivateElementFind(_O_, _P_).
1. If _entry_ is not ~empty~, throw a *TypeError* exception.
1. Append PrivateElement { [[Key]]: _P_, [[Kind]]: ~field~, [[Value]]: _value_ } to _O_.[[PrivateElements]].
Expand All @@ -6834,6 +6836,8 @@ <h1>
</dl>
<emu-alg>
1. Assert: _method_.[[Kind]] is either ~method~ or ~accessor~.
1. If the host is a web browser, then
1. Perform ? HostEnsureCanAddPrivateElement(_O_).
1. Let _entry_ be PrivateElementFind(_O_, _method_.[[Key]]).
1. If _entry_ is not ~empty~, throw a *TypeError* exception.
1. Append _method_ to _O_.[[PrivateElements]].
Expand All @@ -6844,6 +6848,25 @@ <h1>
</emu-note>
</emu-clause>

<emu-clause id="sec-hostensurecanaddprivateelement" type="host-defined abstract operation">
<h1>
HostEnsureCanAddPrivateElement (
_O_: an Object,
): either a normal completion containing ~unused~ or a throw completion
</h1>
<dl class="header">
<dt>description</dt>
<dd>It allows host environments to prevent the addition of private elements to particular host-defined exotic objects.</dd>
</dl>
<p>An implementation of HostEnsureCanAddPrivateElement must conform to the following requirements:</p>
<ul>
<li>If _O_ is not a host-defined exotic object, this abstract operation must return NormalCompletion(~unused~) and perform no other steps.</li>
<li>Any two calls of this abstract operation with the same argument must return the same kind of Completion Record.</li>
</ul>
<p>The default implementation of HostEnsureCanAddPrivateElement is to return NormalCompletion(~unused~).</p>
<p>This abstract operation is only invoked by ECMAScript hosts that are web browsers.</p>
</emu-clause>

<emu-clause id="sec-privateget" type="abstract operation">
<h1>
PrivateGet (
Expand Down Expand Up @@ -48146,6 +48169,16 @@ <h1>Changes to the `typeof` Operator</h1>
</emu-alg>
</emu-annex>
</emu-annex>

<emu-annex id="sec-web-compat-host-make-job-callback">
<h1>Non-default behaviour in HostMakeJobCallback</h1>
<p>The HostMakeJobCallback abstract operation allows hosts which are web browsers to specify non-default behaviour.</p>
</emu-annex>

<emu-annex id="sec-web-compat-host-ensure-can-add-private-field">
<h1>Non-default behaviour in HostEnsureCanAddPrivateElement</h1>
<p>The HostEnsureCanAddPrivateElement abstract operation allows hosts which are web browsers to specify non-default behaviour.</p>
</emu-annex>
</emu-annex>
</emu-annex>

Expand Down