Q:

Trying to apply CSS transform in javascript

On my page I have an element that follows users cursor.
What I'm trying to do is to use transform instead top/left with plain javascript. Without any libraries or depencies...
The problem is that I need to apply values stored in variables to the transform property. I didn't find anything that could help me...
Here is my code:

var cursor = document.getElementById('cursor');
document.body.addEventListener("mousemove", function(e) {
  //storing cursor position as variables
  var curX = e.clientX;
  var curY = e.clientY;
  
  
  // I need the code below to be replaced with transform-translate instead of top/left
  // I can not get this to work with any other method than top/left
  cursor.style.left = curX - 7 + 'px';
  cursor.style.top = curY - 7 + 'px';
    
});
body {
  background: orange;
  height: 500px;
  width: 500px;
}
#cursor {
    position: fixed;
    z-index: 20000;
    height: 14px;
    width: 14px;
    background-color: #222;
    border-radius: 50%;
    pointer-events: none!important;
}
<body>
<div id="cursor"></div>
</body>

It's a simple or even a silly question but I have not found anything like this on stackoverflow or google...

A:

You can do it like below. Don't forget to set top/left to always have the same behavior since translate will apply translation from the position of the element.

var cursor = document.getElementById('cursor');
document.body.addEventListener("mousemove", function(e) {
  //storing cursor position as variables
  var curX = e.clientX;
  var curY = e.clientY;


  // I need the code below to be replaced with transform-translate instead of top/left
  // I can not get this to work with any other method than top/left
  //cursor.style.left = curX - 7 + 'px';
  //cursor.style.top = curY - 7 + 'px';
  cursor.style.transform = "translate(" + (curX - 7) + "px," + (curY - 7) + "px)";

});
body {
  background: orange;
  height: 500px;
  width: 500px;
}

#cursor {
  position: fixed;
  top:0;
  left:0;
  z-index: 20000;
  height: 14px;
  width: 14px;
  background-color: #222;
  border-radius: 50%;
  pointer-events: none!important;
}
<body>
  <div id="cursor"></div>
</body>

You can consider percentage and calc() if you want this to be dynamic and work with any width/height:

var cursor = document.getElementById('cursor');
document.body.addEventListener("mousemove", function(e) {
  //storing cursor position as variables
  var curX = e.clientX;
  var curY = e.clientY;


  // I need the code below to be replaced with transform-translate instead of top/left
  // I can not get this to work with any other method than top/left
  //cursor.style.left = curX - 7 + 'px';
  //cursor.style.top = curY - 7 + 'px';
  cursor.style.transform = "translate(calc(" + curX + "px - 50%),calc(" + curY + "px - 50%))";

});
body {
  background: orange;
  height: 500px;
  width: 500px;
  margin: 0;
}

#cursor {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 20000;
  height: 14px;
  width: 14px;
  background-color: #222;
  border-radius: 50%;
  pointer-events: none!important;
}
<body>
  <div id="cursor"></div>
</body>

