Change the :Before Selector from JavaScript

Change the :before selector from javascript

No, you cannot access :before or :after from javascript, because they are not a part of the DOM. However you can still achieve your goal by using CSS classes:

<script>
    document.getElementById('abc').className = "minus";
</script>

<style>
    #abc:before {content: "+";}
    #abc.minus:before {content: "-"}
</style>

In fact this approach is more unobtrusive, because you don't mix representation with javascript. Tomorrow you might want to change text "+/-" to say nice background images, in this case you don't have to touch javascript code at all.

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';
}

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 change content value of pseudo :before element by Javascript

Update (2018): as has been noted in the comments, you now can do this.

You can't modify pseudo elements through JavaScript since they are not part of the DOM. Your best bet is to define another class in your CSS with the styles you require and then add that to the element. Since that doesn't seem to be possible from your question, perhaps you need to look at using a real DOM element instead of a pseudo one.

modify pseudo select :after in javascript

If that style comes from a CSS file, you'll have to search for it in document.styleSheets, which will be messy.

If you are open to dynamically creating a <style> element containing that CSS instead, you can modify it programmatically.

var slidingTagLiAfterStyle = document.createElement("style");
slidingTagLiAfterStyle.innerHTML =
 ".slidingTag li:after {
    content: '';
    z-index: 3;
    height: 6px;
  }";
document.head.appendChild(slidingTagLiAfterStyle);

...

slidingTagLiAfterStyle.innerHTML = slidingTagLiAfterStyle.innerHTML.replace(/height: [0-9]+px/, "height: 12px"); // or whatever you want to set it to

Manipulate CSS before selector with jQuery

You can't change the style of pseudo elements with javascript, but you could add a <span> inside of your .className and then target that.

$('.className span').css('background-color', bgColor);

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