Ok, found the culprit
code should be

          var newOrderCol = this.getAttribute('data-order');
          var newDirection = this.getAttribute('data-direction');

will make patch

hmm
using npm install with a modified searchtools.es6.js where I have
const newOrderCol = this.getAttribute('data-order');
const newDirection = this.getAttribute('data-direction');

creates a searchtools.js with
var newOrderCol = _this.getAttribute('data-order');
var newDirection = _this.getAttribute('data-direction');

I.e. an underscore is added...

although we have in the es6 file
const self = this;
self does not work either.

@dgrammatiko
I am lost

This should work:

item.addEventListener('click', (event) => {
  // Order to set
  const newOrderCol = event.target.getAttribute('data-order');
  const newDirection = event.target.getAttribute('data-direction');
  const newOrdering = `${newOrderCol} ${newDirection}`;

Obviously this is the class so it won't work on this case as we want to fetch the attributes of the element clicked. And obviously you have to blame me for this ?

That's it. making patch now.

See #21539

Could you reopen here pls. because the pr is closed without merging?

Reopened as requested

Closed as we have patch
#21562