<!DOCTYPE html>
<html>
<body>

<p style="background:red;min-width:600px;" id="myP">The minimum width of this paragraph is set to 600px.</p>

<button type="button" onclick="myFunction()">Return min width</button>

<script>
function myFunction() {
    alert(document.getElementById("myP").style.minWidth);
}
</script>

</body>
</html>