How do I detect that no modifier keys are pressed without manually checking all modifiers?

Question

I am handling keydown which provides a KeyboardEvent on a webpage. I want to only do something if the user pressed a specific letter without holding any modifiers. However, I cannot figure out how to check for this properly.

I can manually check that KeyboardEvent.altKey, KeyboardEvent.ctrlKey, KeyboardEvent.metaKey, and KeyboardEvent.shiftKey are all false. (And I will immediately exit my handler if KeyboardEvent.isComposing is set. That is not exactly a modifier key, but I also do not want to interfere with composition incorrectly.)

However, I think I am missing some guidance here. Is this the exhaustive list of properties to check? What if new keyboards come out with a new modifier key that is somehow super popular and adopted by everyone and even the Web API adds another property for it in the future?

Is there a property which has a bitmask of the modifier keys so I can check for a specific combination of modifiers easily while also being able to easily check that no modifiers are pressed by comparing it to 0? This would match the problem I am trying to solve much better than the split out properties.