Why can't I undo transform:scale() with the formula 1/a?

I am trying to write some JavaScript code that scales a div element using the transform:scale() CSS property, and then un-scales it so that the element returns to its original size. I thought that if I scale the element by applying transform:scale(a), I could un-scale it by applying transform:scale(1/a), since (element x a) x (1 / a) = element. However, that does not seem to work. Why not?

function scale_up() {
  document.getElementById("div").style.transform = "scale(3)";
}

function scale_down() {
  document.getElementById("div").style.transform = "scale(0.33)";
}

setTimeout(scale_up, 3000)

setTimeout(scale_down, 6000) /* this should return the div to its original size, but it doesn't */

#div {
  height: 30px;
  width: 30px;
  border-style: solid;
}

<body>
  <div id="div"></div>
</body>

Solution

CSS is a declarative language, and thus applying new properties on a given element or selector replaces existing ones with the same property name; properties are not added or multiplied when you set them again.

CSS transforms, building upon this, work the same way: setting the transform to scale(3) and then setting it to scale(0.33) has the same effect as just setting it to scale(0.33): the latter transform replaces the former.

Applying this principle, to undo the transform you can simply remove the CSS property that applies it to your element; you can do this by simply setting the property to an empty string, as per this StackOverflow answer. Alternatively, in this case, you can simply set a scale of 1:

function scale_up() {
  document.getElementById("div").style.transform = "scale(3)";
}

function scale_down() {
  document.getElementById("div").style.transform = "";

  // This would also work:
  //document.getElementById("div").style.transform = "scale(1)";
}

setTimeout(scale_up, 3000)

setTimeout(scale_down, 6000)

#div {
  height: 30px;
  width: 30px;
  border-style: solid;
}

<body>
  <div id="div"></div>
</body>