QA Is isogram (uk)

An isogram is a word that has no repeating letters, consecutive or non-consecutive.

Write a tests for the function isIsogram that takes a string word, that contains only letters, and checks whether this word is an isogram.

Notes:

for this task, the empty string is an isogram;
function should be case-insensitive (M and m are considered the same letter).

Examples:

isIsogram('playgrounds') === true
isIsogram('look') === false
isIsogram('Adam') === false
isIsogram('') === true

Read more about Jest expectations.