Jase Jase - 1 month ago 10
CSS Question

Using CSS Variables (custom properties) in a Pseudo-element's "content:" Property

Example use (what I want)

div::after {
content: var(--mouse-x) ' / ' var(--mouse-y);
}


Test case showing it NOT working:

CodePen: CSS Variables in Pseudo Element's "content:" Property (a test case) by Jase Smith



document.addEventListener('mousemove', (e) => {
document.documentElement.style.setProperty('--mouse-x', e.clientX)
document.documentElement.style.setProperty('--mouse-y', e.clientY)

// output for explanation text
document.querySelector('.x').innerHTML = e.clientX
document.querySelector('.y').innerHTML = e.clientY
})

/* what I want!! */
div::after {
content: var(--mouse-x, 245)" / " var(--mouse-y, 327);
}

/* setup and presentation styles */
div::before {
content: 'mouse position:';
}
div {
position: absolute;
top: 0;
left: 0;
transform: translate(calc(var(--mouse-x, 245) * 1px), calc(var(--mouse-y, 327) * 1px));
width: 10em;
height: 10em;
background: #ff3b80;
color: #fff;
display: flex;
flex-flow: column;
align-items: center;
justify-content: center;
border-radius: 100%;
will-change: transform;
}
body {
margin: 2em;
font-family: sans-serif;
}
p {
max-width: 50%;
min-width: 25em;
}

<!-- test case: element with pseudo element -->
<div></div>

<!-- explanation (not test case) -->
<main>
<pre><code>div::after {
content: var(--mouse-x) ' / ' var(--mouse-y);
}</code></pre>
<h1>If this worked...</h1>
<p>
We should see something like this: <b><span class="x">245</span> / <span class="y">327</span></b> updating with the mousemove coordinates inside the pseudo <i>::after</i> element for the div.
</p>
</main>




Answer

Got it to work using a hack involving CSS Counters. Enjoy.

div::after {
  counter-reset: mouse-x var(--mouse-x, 245) mouse-y var(--mouse-y, 245);
  content: counter(mouse-x) " / " counter(mouse-y);
}

Full code in action:

document.addEventListener('mousemove', (e) => {
  document.documentElement.style.setProperty('--mouse-x', e.clientX)
  document.documentElement.style.setProperty('--mouse-y', e.clientY)
  
  // output for explanation text
  document.querySelector('.x').innerHTML = e.clientX
  document.querySelector('.y').innerHTML = e.clientY
})
/* what I want!! */
div::after {
  counter-reset: mouse-x var(--mouse-x, 245) mouse-y var(--mouse-y, 245);
  content: counter(mouse-x) " / " counter(mouse-y);
}

/* setup and presentation styles */
div::before {
  content: 'mouse position:';
}
div {
  position: absolute;
  top: 0;
  left: 0;
  transform: translate(calc(var(--mouse-x, 245) * 1px), calc(var(--mouse-y, 327) * 1px));
  width: 10em;
  height: 10em;
  background: #ff3b80;
  color: #fff;
  display: flex;
  flex-flow: column;
  align-items: center;
  justify-content: center;
  border-radius: 100%;
  will-change: transform;
}
body {
  margin: 2em;
  font-family: sans-serif;
}
p {
  max-width: 50%;
  min-width: 25em;
}
<!-- test case: element with pseudo element -->
<div></div>

<!-- explanation (not test case) -->
<main>
  <pre><code>div::after {
  content: var(--mouse-x) ' / ' var(--mouse-y);
}</code></pre>
  <h1>If this worked...</h1>
  <p>
    We should see something like this: <b><span class="x">245</span> / <span class="y">327</span></b> updating with the mousemove coordinates inside the pseudo <i>::after</i> element for the div.
  </p>
</main>

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