add overlay using javascript code example

Example: create overlay js

<body>
  <div id="#overlay">
    
  </div>
  <div> content </div>
</body>

//first create an element with folowing styles
#overlay {
  position: fixed; /* Sit on top of the page content */
  display: none; /* Hidden by default */
  width: 100%; /* Full width (cover the whole page) */
  height: 100%; /* Full height (cover the whole page) */
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0,0,0,0.5); /* Black background with opacity */
  z-index: 2; /* Specify a stack order in case you're using a different order for other elements */
  cursor: pointer; /* Add a pointer on hover */
}

//then you can do
const on = () => {
  document.getElementById("overlay").style.display = "block";
}

const off = () => {
  document.getElementById("overlay").style.display = "none";
}