Document.fullscreenElement

The Document.fullscreenElement read-only property returns the Element that is currently being presented in full-screen mode in this document, or null if full-screen 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.

document.fullscreenElement

The Element object that's currently in full-screen mode; if full-screen 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 full-screen mode (fullscreenElement isn't null) and the full-screen element's nodeName is VIDEO, indicating a <video> element, the function returns true, indicating that the video is in full-screen mode.

function isVideoInFullscreen() {
  if (document.fullscreenElement && 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