I would have expected to see the hasProperty check here, or as a check of its own.

Why not just return ScriptInterface::FromJSVal(cx, value, out);?

Do you mean:

if (!JS_HasProperty(cx, obj, name, &hasProperty) || !hasProperty)
    ...

?
hasProperty is just checked below.

True, just used style from other ScriptConversions functions.

Yes. But why is it checked there, doesn't seem like the place where one would expect that.

Just adding another if (!hasProperty) return false; would be ok too.

Quite a few of those are strange in more than that way, would be nice to improve that if we are adding new ones.

No problem, but all people have different vision on style, even in a short period.

( https://code.wildfiregames.com/D338?id=1265#inline-6502 )

Not sure what you'd want to extend there, since at that point there is nothing left to do. Any change that would occur there would most likely need to change something else too, and that should be done if we need it (which I doubt).