Re: [heycam/webidl] Modernize invoking user code (#113)

>                <ol>
> -                <li>If the callback function has a <a class='dfnref' href='#dfn-return-type'>return type</a> that is a <a href='#idl-promise'>promise type</a>, then:
> +                <li>If <var>arg</var><sub><var>i</var></sub> is the special value “missing”, then

Ah.  "missing" is what you get as the IDL value when `undefined` is passed to an optional argument with no default value.  It's handled in ES-to-IDL conversion for method calls, and for calls into a callback function; just not for the user object operation case.

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