why this erorr Uncaught TypeError: Cannot set property 'className' of undifined Line 6

Question

why this error can happen when this is exactly the same as in the example
https://code.sololearn.com/W05OhQU2k4Kb/?ref=app
and this is an example : https://codepen.io/SitePoint/pen/KzjpGj

Answer 1

Because your code runs prior to the webpage being loaded, slides is null. This adding onload function fixes it.

https://code.sololearn.com/Wdz7LE4T129V

Answer 2

John Wells ok the code is working as I want thanks for helping me overcome this erorr