var width = document.getElementById('myID').offsetWidth;//includes margin,border,padding
var height = document.getElementById('myID'). offsetHeight;//includes margin,border,padding
document.getElementById("myElID").style.width = "100px";
var intElemOffsetHeight = element.offsetHeight;
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
</head>
<body>
<div id="box" style="background-color: salmon">Box 1</div>
<script src="index.js"></script>
</body>
</html>