Setting CSS Pseudo-Class Rules from JavaScript

How to change a Pseudo-class style through JavaScript?

I would use a different set of rules with an additional class

<html>
<head>
  <style type='text/css'>
    #myBtn {
      background-color: red;
    }
    #myBtn:hover {
      background-color: blue;
    }
    #myBtn.changed {
      background-color: green;
    }
    #myBtn.changed:hover {
      background-color: yellow; /* or whatever you want here */
    }
</head>

And then

<script type="application/x-javascript">
      function ChangeColor() {
         var btn = document.getElementById('myBtn');
         btn.className = "changed";
      };
</script>

Of course, this makes only sense for one or a couple of different values you want to have for your color.

Changing CSS pseudo-element styles via JavaScript

EDIT: There is technically a way of directly changing CSS pseudo-element styles via JavaScript, as this answer describes, but the method provided here is preferable.

The closest to changing the style of a pseudo-element in JavaScript is adding and removing classes, then using the pseudo-element with those classes. An example to hide the scrollbar:

CSS

.hidden-scrollbar::-webkit-scrollbar {
   visibility: hidden;
}

JavaScript

document.getElementById("editor").classList.add('hidden-scrollbar');

To later remove the same class, you could use:

document.getElementById("editor").classList.remove('hidden-scrollbar');

Selecting and manipulating CSS pseudo-elements such as ::before and ::after using javascript (or jQuery)

You could also pass the content to the pseudo element with a data attribute and then use jQuery to manipulate that:

In HTML:

<span>foo</span>

In jQuery:

$('span').hover(function(){
    $(this).attr('data-content','bar');
});

In CSS:

span:after {
    content: attr(data-content) ' any other text you may want';
}

If you want to prevent the 'other text' from showing up, you could combine this with seucolega's solution like this:

In HTML:

<span>foo</span>

In jQuery:

$('span').hover(function(){
    $(this).addClass('change').attr('data-content','bar');
});

In CSS:

span.change:after {
    content: attr(data-content) ' any other text you may want';
}

How to change css pseudo-element through javascript?

Well I finally did it, I don't know if what I did is good programming but anyway it works, I needed to add a css rule so by doing this:

document.styleSheets[0].insertRule('input[type=range]::-webkit-slider-thumb 
{ background-color:' + color + ' !important; }', 0);

I was able to finally manipulate the pseudo-element (at least in chrome, next step other major browsers).
Anyway I think this is helpful to someone who wants to try something like this, I needed to search and tried many things to do this.

Thanks to everyone who tried to help me.

Changing pseudo-element style from javascript

Since pseudo-elements do not exist in the DOM, they cannot be accessed in Javascript.
The workaround is to create a <span> instead of using :before and the same logic has to be applied.

How do you add pseudo classes to elements using jQuery?

You can't force a certain pseudo-class to apply using jQuery. That's not how pseudo-classes (especially dynamic pseudo-classes) work, since they're designed to select based on information that cannot be expressed via the DOM (spec).

You have to specify another class to use, then add that class using jQuery instead. Something like this:

.element_class_name:hover, .element_class_name.jqhover {
    /* stuff here */
}

$(this).addClass("jqhover");

Alternatively you could hunt for the .element_class_name:hover rule and add that class using jQuery itself, without hardcoding it into your stylesheet. It's the same principle, though.

Dynamically styling pseudo-elements using jQuery or Javascript

May I suggest doing like this, where you dynamically create a style element (with a unique id so you can access it over and over) and just update a rule repeatedly

Usage

loadStyle('p.clicker::after { margin-top: ' + calculated-value + 'px; }');

Script

function loadStyle(css) {
  var style = document.querySelector('style[id="lastinbody"]') || document.createElement('style');
  style.id = 'lastinbody';
  style.type = 'text/css';
  if (style.styleSheet){
    style.styleSheet.cssText = css;
  } else {
    style.appendChild(document.createTextNode(css));
  }
  document.querySelector('body').appendChild(style);
}

At this post of mine is a few more versions (which might be consider as a possible duplicate)