Changing CSS "z-index" Property: "Invalid Left-hand Side In Assignment"

I have an element with the onmouseover event handler which calls a function I made that is suppose to change the css property of another element and I gave that element the ID 'out

Solution 1:

Style names in Javascript use camelCase, so it should be:

document.getElementById("output").style.zIndex="-1";

- is the arithmetic subtraction operator, it can't used in identifiers.

Solution 2:

You forgot a quote, and hyphens aren't allowed in property names, so usually camelcase is used instead

<script>
function ChangeZofO () {
    document.getElementById("output").style.zIndex = -1;
}
</script>

<div id="MyDiv" onmouseover="ChangeZofO()"></div>