How to read width of css element with javascript?

Question

So I want this VarDisk2 element to disappear after VarDisk1 width reaches more than 70. How can I do this?

 <script>
        var VarDisk1 = document.querySelector("Disk1");
        var VarDisk2 = document.getElementsByClassName("Disk2");
        let width = VarDisk1.offsetWidth;

        if(width > 70)
            VarDisk2.style.display = "none";

    </script>

Answer 1

For checking all classes

<script>
        ...
        var VarDisk2 = document.getElementsByClassName("Disk2");
        let width = VarDisk1.offsetWidth;

        if(width > 70) {
            VarDisk2.forEach(ele => ele.style.display = "none";)
        }

</script>

for first class:

<script>
        ...
        var VarDisk2 = document.getElementsByClassName("Disk2")[0];
        ...
</script>