for own..in

[edemaine]

Yet another feature from CoffeeScript for loops:

for own key in object
↓↓↓
for (key in object) {
  if (!Object.hasOwn(object, key)) continue;

I also switched from Object.prototype.hasOwnProperty to Object.hasProp (still without using Object as an explicit name),
as recommended in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Also fixed up types some around this code (but there are still errors).

[STRd6]

Looks good, nice to see the type improvements too!