It's not really good idea to just copy paste code in codepen when asking for help on forum. For example, you have posted images which are not present, the next and previous buttons are labeled as test1, test2 and most importantly your slider wasn't even visible to see what and if anything is happening.   Anyway, nothing was working because you copied exact same code for next button and renamed it for previous button and click event was created on wrong id. I fixed that and