How to map function on each element with css class in javascript

I'm rewriting some stuff which I created with jQuery to vanilla JS. I can't find vanilla JS equivalent for mapping my function to all elements with specified CSS class. For example I have function for slider, in jQuery I'm mapping it like this:

(function($) {

    $.fn.slider = function() {
        this.each(function() {
            // Do some stuff
        });
    };

    $('.slider').slider();

}(jQuery));

What is the propper way to do it in vanilla JS?

Implement e.g. a function initializeSlider which accepts an element-node parameter and do whatever has to be done with such a node.

Query a NodeList of all .slider classified element-nodes via e.g. querySelectorAll and execute the slider related function forEach of the node-list's node-items.

                                      // (function($) {
                                      //
                                      //   $.fn.slider = function() {
function initializeSlider(elmNode) {  //     this.each(function() {
  // Do some stuff with/to `elmNode`  //       // Do some stuff
}                                     //     });
document                              //   };
  .querySelectorAll('.slider')        //
  .forEach(initializeSlider);         //   $('.slider').slider();
                                      //
                                      // }(jQuery));