This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The getActive() method of the HTMLIFrameElement indicates whether the current browser <iframe> is the currently active frame.
var amIActive = instanceOfHTMLIframeElement.getActive();
A Boolean indicating whether the current browser <iframe> is the currently active frame (true) or not (false.)
var browser = document.querySelector('iframe');
var amIActive = browser.getActive();