Is this 6kuy kata?

good solution, but it doesn't check for alpha, now we can pass not only letters!

For example this call in this solution findMissingLetter(['<','>']) return us '=' which is not a letter actually

You don't need to reassign the array variable like this 'array = array.sort', because sort() is already mutating the array, also be afraid that it will mutate the initial array from the argument, hence better to do something like this 'const innerArray = [...array] and work with it.

Be careful. This solution will change the initial array.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
"Note that the array is sorted in place, and no copy is made."

Oh, I am not alone in using subtraction instead of division :))

How did u find solution like that?