You just linked the general solution page. I think you wanted to link your solution instead.

Your solution includes multiple loops, just not by yourself but by called functions. includes(), indexOf(), slice() and string concatenation all loop over the strings again and again and also create new string objects every time. Since you call them in a loop your solution is O(n²) while this solution loops over each string exactly once and once over the stack, which contains at most 26 elements.

Leave it, maybe it helps someone.

Your task is to return the first longest string

Two strings are the longest: "folingtrashy" and "abcdefuvwxyz".
The first that came is "folingtrashy" so 
longest_consec(strarr, 2) should return "folingtrashy".

It's explained in the description.

A person of principles (+1 for not using eval ^^)