get div one height and apply margin top to below div jquery code example

Example 1: javascript get element width

var width = document.getElementById('myID').offsetWidth;//includes margin,border,padding
var height = document.getElementById('myID'). offsetHeight;//includes margin,border,padding

Example 2: how to horizontally center header at the top of page with flexbox css

.box.flex {
  display: flex;
  justify-content: center;
}