Changing pseudo-element style from javascript [duplicate]

Answer 1

Here are two ways to directly manipulate a pseudo-element:

First way, is by using some sort of style manager.
This "manager" is an object with methods which allows easier manipulation of CSS rules on-the-fly, so here is a very basic example which you can study and implement for your specific needs:

var elm = document.querySelector('main');

// Reference: https://stackoverflow.com/a/28930990/104380
var styleManager = (function() {
    // Create the <style> tag
    var style = document.createElement("style")

    // WebKit hack
    style.appendChild(document.createTextNode(""));

    // Add the <style> element to the page
    document.head.appendChild(style);
    
    function getStyleRuleIndexBySelector(selector, prop){
      var result = [], i,
          value = (prop ? selector + "{" + prop + "}" : selector).replace(/\s/g, ''), // remove whitespaces
          s = prop ? "cssText" : "selectorText";

      for( i=0; i < style.sheet.cssRules.length; i++ )
        if( style.sheet.cssRules[i][s].replace(/\s/g, '') == value)
          result.push(i);

      return result;
    };

    return {
      style : style,
      
      getStyleRuleIndexBySelector : getStyleRuleIndexBySelector,
      
      add(prop, value){
        return style.sheet.insertRule(`${prop}{${value}}`, style.sheet.cssRules.length);
      },
      
      remove(selector, prop){
        var indexes = getStyleRuleIndexBySelector(selector, prop), i = indexes.length;
        
        // reversed iteration so indexes won't change after deletion for each iteration
        for( ; i-- ; )
          style.sheet.deleteRule( indexes[i] );
      }
    }
})();


elm.addEventListener('mouseenter', function(){
   // each new rule should be added the END of the sheet
   styleManager.add('main::before','left:90%; top:60%;');
   styleManager.add('main::before','left:70%;');
});

elm.addEventListener('mouseleave', function(){
  styleManager.remove('main::before', 'left:70%;');  // you can also try without the "left:70%;" part
});

main{  
  position:relative;
  width: 200px;
  height: 200px;
  border: 1px dashed silver;
}

main::before{
  content: 'pseudo';
  width: 100px;
  height: 100px;
  background: lightgreen;
  position: absolute;
  left: 10px;
  top: 40px;
  
  transition:.3s ease-out;
}

<main>Hover &amp; out</main>

Another way - with CSS variables:

var elm = document.querySelector('main');

elm.addEventListener('mouseenter', function(){
   elm.style.setProperty('--before-left', '90%');
});

elm.addEventListener('mouseleave', function(){
  elm.style.setProperty('--before-left', '10px');
});

main{  
  --before-left : 10px; /* <-- Your CSS should use variables for this to work */
  
  position:relative;
  width: 200px;
  height: 200px;
  border: 1px dashed silver;
}

main::before{
  content: 'pseudo';
  width: 100px;
  height: 100px;
  background: lightgreen;
  position: absolute;
  left: var(--before-left); /* <-- using the variable */
  top: 40px;

  transition:.3s ease-out;
}

<main>Hover &amp; out</main>

Answer 2

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.