Change :hover CSS properties with JavaScript

You can't change or alter the actual :hover selector through Javascript. You can, however, use mouseenter to change the style, and revert back on mouseleave (thanks, @Bryan).

Pretty old question so I figured I'll add a more modern answer. Now that CSS variables are widely supported they can be used to achieve this without the need for JS events or !important.

Taking the OP's example:

<table>
  <tr>
    <td>Hover 1</td>
    <td>Hover 2</td>
  </tr>
</table>

We can now do this in the CSS:

table td:hover {
  // fallback in case we need to support older/non-supported browsers (IE, Opera mini)
  background: #ff0000;
  background: var(--td-background-color);
}

And add the hover state using javascript like so:

const tds = document.querySelectorAll('td');
tds.forEach((td) => {
  td.style.setProperty('--td-background-color', '#00ff00');
});

Here's a working example https://codepen.io/ybentz/pen/RwPoeqb

What you can do is change the class of your object and define two classes with different hover properties. For example:

.stategood_enabled:hover  { background-color:green}
.stategood_enabled        { background-color:black}
.stategood_disabled:hover { background-color:red}
.stategood_disabled       { background-color:black}

And this I found on: Change an element's class with JavaScript

function changeClass(object,oldClass,newClass)
{
    // remove:
    //object.className = object.className.replace( /(?:^|\s)oldClass(?!\S)/g , '' );
    // replace:
    var regExp = new RegExp('(?:^|\\s)' + oldClass + '(?!\\S)', 'g');
    object.className = object.className.replace( regExp , newClass );
    // add
    //object.className += " "+newClass;
}

changeClass(myInput.submit,"stategood_disabled"," stategood_enabled");

Sorry to find this page 7 years too late, but here is a much simpler way to solve this problem (changing hover styles arbitrarily):

HTML:

<button id=Button>Button Title</button>

CSS:

.HoverClass1:hover {color: blue !important; background-color: green !important;}
.HoverClass2:hover {color: red !important; background-color: yellow !important;}

JavaScript:

var Button=document.getElementById('Button');
/* Clear all previous hover classes */
Button.classList.remove('HoverClass1','HoverClass2');
/* Set the desired hover class */
Button.classList.add('HoverClass1');

If it fits your purpose you can add the hover functionality without using css and using the onmouseover event in javascript

Here is a code snippet

<div id="mydiv">foo</div>

<script>
document.getElementById("mydiv").onmouseover = function() 
{
    this.style.backgroundColor = "blue";
}
</script>

You can use mouse events to control like hover. For example, the following code is making visible when you hover that element.

var foo = document.getElementById("foo");
foo.addEventListener('mouseover',function(){
   foo.style.display="block";
})
foo.addEventListener('mouseleave',function(){
   foo.style.display="none";
})

I'd recommend to replace all :hover properties to :active when you detect that device supports touch. Just call this function when you do so as touch()

   function touch() {
      if ('ontouchstart' in document.documentElement) {
        for (var sheetI = document.styleSheets.length - 1; sheetI >= 0; sheetI--) {
          var sheet = document.styleSheets[sheetI];
          if (sheet.cssRules) {
            for (var ruleI = sheet.cssRules.length - 1; ruleI >= 0; ruleI--) {
              var rule = sheet.cssRules[ruleI];

              if (rule.selectorText) {
                rule.selectorText = rule.selectorText.replace(':hover', ':active');
              }
            }
          }
        }
      }
    }

This is not actually adding the CSS to the cell, but gives the same effect. While providing the same result as others above, this version is a little more intuitive to me, but I'm a novice, so take it for what it's worth:

$(".hoverCell").bind('mouseover', function() {
    var old_color = $(this).css("background-color");
    $(this)[0].style.backgroundColor = '#ffff00';

    $(".hoverCell").bind('mouseout', function () {
        $(this)[0].style.backgroundColor = old_color;
    });
});

This requires setting the Class for each of the cells you want to highlight to "hoverCell".

You can make a CSS variable, and then change it in JS.

:root {
  --variableName: (variableValue);
}

to change it in JS, I made these handy little functions:

var cssVarGet = function(name) {
  return getComputedStyle(document.documentElement).getPropertyValue(name);
};

and

var cssVarSet = function(name, val) {
  document.documentElement.style.setProperty(name, val);
};

You can make as many CSS variables as you want, and I haven't found any bugs in the functions; After that, all you have to do is embed it in your CSS:

table td:hover {
  background: var(--variableName);
}

And then bam, a solution that just requires some CSS and 2 JS functions!

I had this need once and created a small library for, which maintains the CSS documents

https://github.com/terotests/css

With that you can state

css().bind("TD:hover", {
        "background" : "00ff00"
    });

It uses the techniques mentioned above and also tries to take care of the cross-browser issues. If there for some reason exists an old browser like IE9 it will limit the number of STYLE tags, because the older IE browser had this strange limit for number of STYLE tags available on the page.

Also, it limits the traffic to the tags by updating tags only periodically. There is also a limited support for creating animation classes.

Declare a global var:

var td

Then select your guiena pig <td> getting it by its id, if you want to change all of them then

window.onload = function () {
  td = document.getElementsByTagName("td"); 
}

Make a function to be triggered and a loop to change all of your desired td's

function trigger() {
    for(var x = 0; x < td.length; x++) { 
      td[x].className = "yournewclass"; 
    }
}

Go to your CSS Sheet:

.yournewclass:hover { background-color: #00ff00; }

And that is it, with this you are able to to make all your <td> tags get a background-color: #00ff00; when hovered by changing its css propriety directly (switching between css classes).

Had some same problems, used addEventListener for events "mousenter", "mouseleave":

let DOMelement = document.querySelector('CSS selector for your HTML element');

// if you want to change e.g color: 
let origColorStyle = DOMelement.style.color;

DOMelement.addEventListener("mouseenter", (event) => { event.target.style.color = "red" });

DOMelement.addEventListener("mouseleave", (event) => { event.target.style.color = origColorStyle })

Or something else for style when cursor is above the DOMelement. DOMElement can be chosen by various ways.

For myself, I found the following option: from https://stackoverflow.com/a/70557483/18862444

const el = document.getElementById('elementId');
el.style.setProperty('--focusHeight', newFocusHeight);
el.style.setProperty('--focusWidth', newFocusWidth);

.my-class {
  --focusHeight: 32px;
  --focusWidth: 256px;
}

.my-class:focus {
  height: var(--focusHeight);
  width: var(--focusWidth);
}

I was researching about hover, to be able to implement them in the button label and make the hover effect

<button type="submit"
        style=" background-color:cornflowerblue; padding:7px; border-radius:6px"
        onmouseover="this.style.cssText ='background-color:#a8ff78; padding:7px; border-radius:6px;'"
        onmouseout="this.style.cssText='background-color:cornflowerblue; padding:7px; border-radius:6px'"
        @click="form1()">
        Login
</button>

    const tds = document.querySelectorAll('td');
    tds.forEach((td,index) => {
      td.addEventListener("mouseover", ()=>hover(index))
      td.addEventListener("mouseout", ()=>normal(index))
    });
    function hover(index){
      tds[index].style.background="red";
    }
    function normal(index){
      tds[index].style.background="yellow";
    }

Try this code it will work fine .

You can create a class in css

.hover:hover {
    background: #ff0000;
}

and then add it dynamically

const columns = document.querySelectorAll('table td');

for (let i = 0; i < columns.length; i++) {
   columns[i].classList.add('hover');
}

But your css and js files should be connected in index.html

If you use lightweight html ux lang, check here an example, write:

div root
 .onmouseover = ev => {root.style.backgroundColor='red'}
 .onmouseleave = ev => {root.style.backgroundColor='initial'}

The code above performes the css :hover metatag.