Instructions:
An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.

Example: (Input --> Output)

"Dermatoglyphics" --> true
"aba" --> false
"moOse" --> false (ignore letter case)

Thoughts:

1. Converting the string to lowercase to ensure case insensitivity.
2. Using nested loops to compare each character with every subsequent character.
3. Returning false immediately if a duplicate character is found.
4. Returning true if no duplicates exist after checking all characters.

This is a CodeWars Challenge of 7kyu Rank (https://www.codewars.com/kata/54ba84be607a92aa900000f1/solutions/javascript)