That is an extra fail-safe. In the chance that strarr.each_cons(k).map(&:join).max_by(&:length) returns nil, which it will if the array strarr is empty, then an empty string will be returned. Since nil is falsey, '' (the empty string) will be returned when the || expression is evaluated on the last line—since strings are considered truthy. The first line of the method should handle cases where strarr points to an empty string, but it's not bad to have a back up.

what is the purpose of || "" at the end?

nice catch

Neat! Can you explain your logic here?

The last few characters of this solution are unnecessary.

Works fine without || "" at the end of second line.

It took me a second, but this is brilliant.