The fact that this question even gets asked is an indication that this solution is less than ideal.

What an es5 monstrosity

This solution has a time complexity of O(n^2), which is considered horrible.

the solution is from 8 years ago

why still using var?

why would you do that ?

Because in the second for loop we dont want to compare the same letter to itself.
Also, there is no need to compare the second letter to the first, third letter to the first and second, and so on, if youve already compared them before ever reaching to that particular letter.

i.e
[a, b, c, d , e ,a]
starting from a, you compare it to everything but itself.
when youre at b, you dont need to compare it to a since you did that already. This continues for each letter. Makes sense?

Study Big O Notation.. It's not as hard

How can I measure the O( ) of my code?

I have just tested this code, it doesn't work for the first 2 cases.

Nested loop

this is O(n^2) not a very performant solution.

This isn't a very performanent solution...n^2