For reasons outlined in bug 1450827, the DOM peers have decided to
deprecate support for JS-implemented WebIDL APIs. This means that new
additions of |JSImplementation="foo"| are no longer permitted.

If you maintain an existing JS WebIDL implementation, or are considering
designs for a new API, please get in touch with me or another DOM peer to
discuss alternatives.

Cheers,
bholley

Also sprach Bobby Holley:

> For reasons outlined in bug 1450827, the DOM peers have decided to
> deprecate support for JS-implemented WebIDL APIs. This means that
> new additions of |JSImplementation="foo"| are no longer permitted.

Out of curiosity, and mostly because my knowledge of how this works
is limited, this wouldn’t affect C++ implementations that internally
do_GetService to some XPCOM service implemented in JS?

No, it wouldn't. C++ doesn't really know that a service is implemented in JS.

On Tue, Apr 24, 2018 at 07:25:18AM +0100, Andreas Tolfsen wrote:
>Also sprach Bobby Holley:
>
>> For reasons outlined in bug 1450827, the DOM peers have decided to
>> deprecate support for JS-implemented WebIDL APIs. This means that
>> new additions of |JSImplementation="foo"| are no longer permitted.
>
>
>Out of curiosity, and mostly because my knowledge of how this works
>is limited, this wouldn’t affect C++ implementations that internally
>do_GetService to some XPCOM service implemented in JS?

Correct, it would not affect those implementations.