JavaScript Armstrong Number Validator (ES6)
Being called isArmstrong, it should just return a boolean value, and not have side effects of setting any sort of DOM. That can be done in a separate function by the caller. 
expandedInputValue can probably have a better name. Maybe digitTotal or digitCubeTotal. But the current name isn't helpful.
I think this is a no-op: .map(digit =&gt; digit)