This is what I ended up with, after looking at other solutions and refactoring.

There is a test case necessary, so uppercase and lowercase letters aren't allowed as individual letters:

"AaBbCcDdEeFfGgHhIiJjKkLlMm"

Set is going to include both uppercase and lowercase letters. As an example, this string will cause the function to return true:
"AaBbCcDdEeFfGgHhIiJjKkLlMm"

The array conversion is unnecessary:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set#Relation_with_Strings