Note for readers, charAt will always give you '' when you empty string and also with slice it will give you ''. This helps to reduce the if check

Because the constructor of Array takes the number of items in the list and Array.join joins the items together. This means that when you have 2 items you will only get 1 joined item. As seen in the following example:

[1,2].join('-');

This will return '1-2'. In the solution we create an empty array and join it with the letter. So if n is 2 and we join it without adding 1 we will get 'a' instead of 'aa'.

Why n+1?