I dont think so, given that there has to be a missing letter at index 1 or length -1. In that case it is fine and works. In general it could be better code practice to avoid index errors.

You can omit accessing the element (with [0]) of the array that is returned by the slice method. Less or equal condition coerces that array to a number (when there is not more than one element in an array, as in here).

Off-by-one error. The condition in the loop needs to be i < letters.length - 1.

It's always good to be consistent though.