Document: hasFocus() method

hasFocus()

<p id="log">Focus check results are shown here.</p>
<button id="newWindow">Open new window</button>

body {
  padding: 1rem;
  background: gray;
  text-align: center;
  font: 1.5rem sans-serif;
}

const body = document.querySelector("body");
const log = document.getElementById("log");

function checkDocumentFocus() {
  if (document.hasFocus()) {
    log.textContent = "This document has focus.";
    body.style.background = "white";
  } else {
    log.textContent = "This document does not have focus.";
    body.style.background = "gray";
  }
}

function openWindow() {
  window.open(
    "https://developer.mozilla.org/",
    "MDN",
    "width=640,height=320,left=150,top=150",
  );
}

document.getElementById("newWindow").addEventListener("click", openWindow);
setInterval(checkDocumentFocus, 300);