如何判断当前窗口是否激活

Current browser support:

Chrome 13
Internet Explorer 10
Firefox 10
Opera 12.10 [read notes]
The following code makes use of the API, falling 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";
}
})();