The Document.fullscreenElement read-only property returns the Element that is currently being presented in fullscreen mode in this document, or null if fullscreen mode is not currently in use.

Although this property is read-only, it will not throw if it is modified (even in strict mode); the setter is a no-operation and it will be ignored.

The Element object that's currently in fullscreen mode; if fullscreen mode isn't currently in use by the document, the returned value is null.

This example presents a function, isVideoInFullscreen(), which looks at the value returned by fullscreenElement; if the document is in fullscreen mode (fullscreenElement isn't null) and the fullscreen element's nodeName is VIDEO, indicating a <video> element, the function returns true, indicating that the video is in fullscreen mode.

function isVideoInFullscreen() {
  if (document.fullscreenElement?.nodeName === "VIDEO") {
    return true;
  }
  return false;
}

• Fullscreen API
• Guide to the Fullscreen API
• Element.requestFullscreen()
• Document.exitFullscreen()
• :fullscreen and ::backdrop
• The <iframe> allowfullscreen attribute