Question

I have JavaScript that is doing activity periodically. When the user is not looking at the site (i.e., the window or tab does not have focus), it’d be nice to not run.

Is there a way to do this using JavaScript?

My reference point: Gmail Chat plays a sound if the window you’re using isn’t active.

Answer 1

Since originally writing this answer, a new specification has reached recommendation status thanks to the W3C. The Page Visibility API (on MDN) now allows us to more accurately detect when a page is hidden to the user.

document.addEventListener("visibilitychange", onchange);

Current browser support:

• Chrome 13+
• Internet Explorer 10+
• Firefox 10+
• Opera 12.10+ ^{[read notes]}

The following code falls back to the less reliable blur/focus method in incompatible browsers:

(function() {
  var hidden = "hidden";

  // Standards:
  if (hidden in document)
    document.addEventListener("visibilitychange", onchange);
  else if ((hidden = "mozHidden") in document)
    document.addEventListener("mozvisibilitychange", onchange);
  else if ((hidden = "webkitHidden") in document)
    document.addEventListener("webkitvisibilitychange", onchange);
  else if ((hidden = "msHidden") in document)
    document.addEventListener("msvisibilitychange", onchange);
  // IE 9 and lower:
  else if ("onfocusin" in document)
    document.onfocusin = document.onfocusout = onchange;
  // All others:
  else
    window.onpageshow = window.onpagehide
    = window.onfocus = window.onblur = onchange;

  function onchange (evt) {
    var v = "visible", h = "hidden",
        evtMap = {
          focus:v, focusin:v, pageshow:v, blur:h, focusout:h, pagehide:h
        };

    evt = evt || window.event;
    if (evt.type in evtMap)
      document.body.className = evtMap[evt.type];
    else
      document.body.className = this[hidden] ? "hidden" : "visible";
  }

  // set the initial state (but only if browser supports the Page Visibility API)
  if( document[hidden] !== undefined )
    onchange({type: document[hidden] ? "blur" : "focus"});
})();

onfocusin and onfocusout are required for IE 9 and lower, while all others make use of onfocus and onblur, except for iOS, which uses onpageshow and onpagehide.