Cool! I workaround for the fact that Ruby has no undefined type distinct from nil.

Yes! An illustration in ruby:

main > h = {key_with_nil_value: nil}
=> {:key_with_nil_value=>nil}

main > h[:key_with_nil_value]
=> nil

main > h[:missing_key]  # this would be undefined in JavaScript (a useful distinction) rather than null, but in Ruby it's indistinguishable from the case where a nil value was actually explicitly _supplied_ by the caller/user
=> nil

# so we have to check for "missingness" ("undefinedness"?) differently in Ruby

main > h.key?(:key_with_nil_value)
=> true

main > h.key?(:missing_key)
=> false

This is one unfortunate side effect of Ruby having only nil and no built-in way to distinguish between null and undefined like in JavaScript.