I don't think Web IDL defines this properly, so I filed https://github.com/heycam/webidl/issues/106. But, for the sake of interop with Firefox and Safari, and for consistency with the JS built-ins, Web IDL functions should act like the JS built-ins do:

- No .prototype property
- No [[Construct]] unless otherwise specified (this is different than just having a throwing constructor; the difference is detectable through using proxies)
- No .arguments or .caller properties