在移动端上，JS如何能知道浏览器已经被切换到后台了？

var lastFired = new Date().getTime();
setInterval(function() {
    now = new Date().getTime();
    if(now - lastFired > 5000) { // if it's been more than 5 seconds
        alert("onfocus");
    }
    lastFired = now;
}, 500);