Untitled

In a browser is there a way to find out whether the page is currently visible?
(function () {

    var requiredResolution = 10; // ms
    var checkInterval = 1000; // ms
    var tolerance = 20; // percent


    var counter = 0;
    var expected = checkInterval / requiredResolution;
    //console.log('expected:', expected);

    window.setInterval(function () {
        counter++;
    }, requiredResolution);

    window.setInterval(function () {
        var deviation = 100 * Math.abs(1 - counter / expected);
        // console.log('is:', counter, '(off by', deviation , '%)');
        if (deviation > tolerance) {
            console.warn('Timer resolution not sufficient!');
        }
        counter = 0;
    }, checkInterval);

})();

if (/*@cc_on!@*/false) { // check for Internet Explorer
    document.onfocusin = onFocus;
    document.onfocusout = onBlur;
} else {
    window.onfocus = onFocus;
    window.onblur = onBlur;
}