JS: Node v12 should be used along with its appropiate assertion tools (Mocha + Chai).
Refer to this and this

JS: Should have more sample tests

Mistake in sample tests in Ruby:

Test.assert_equals(insert_missing_letters("hello"),"hIJKMNPQRSTUVWXYZeFGIJKMNPQRSTUVWXYZlMNPQRSTUVWXYZloPQRSTUVWXYZ")​    <--    unknown character after closing parenthesis
                                                                                                                               (it's not being displayed here for some reason)

The empty string has no occurrences of characters. So returning the full aphabet is illogical; returning the empty string would make much more sense. Current specification is in contradiction with the description itself.

Also, it's not even specified. Being a special case, it should definitely be in the description. ( Or expect ""; then it conforms to the existing specification. )