Codewars javascript - Find the missing letter

Steps I took to complete this Javascript challenge:

1) Create an array of all the letters in the english alphabet

2) Determine whether the string is lower or upper case and change the alphabet array accordingly.

3) Find the index of the first letter of the input array in the alphabet.

4) Cut out a piece of the array starting from the index of the first item from the input array and ending on the last character of the input array.

5) Determine which letter is missing by comparing the two arrays together.
To complete this challenge I used:

[clintonwambugu]

Hi, tried your way but I get this error:Expected: 'e', instead got: undefined