The code snippet in the Fiddle is setting a listener on #test, however, the element actually has an id of #test1. This discrepancy needs to be resolved first. After that...
When using
document.getElementsByClassName(...)
, it returns a collection. If your intention is to set the
className = "show" on each element with the class name of
k-grid-header, you will need to iterate over them.
The easiest way to achieve this is by utilizing querySelectorAll() because it provides a collection with a forEach() method for iteration:
document.querySelectorAll('.k-grid-header').forEach(el=>{
el.className = "";
//... etc
});