typescript 判断浏览器前后台的切换监听

private getPage() {
            var hidden, state, visibilityChange;            if (typeof document["hidden"] !== "undefined") {                hidden = "hidden";                visibilityChange = "visibilitychange";                state = "visibilityState";            } else if (typeof document["mozHidden"] !== "undefined") {                hidden = "mozHidden";                visibilityChange = "mozvisibilitychange";                state = "mozVisibilityState";            } else if (typeof document["msHidden"] !== "undefined") {                hidden = "msHidden";                visibilityChange = "msvisibilitychange";                state = "msVisibilityState";            } else if (typeof document["webkitHidden"] !== "undefined") {                hidden = "webkitHidden";                visibilityChange = "webkitvisibilitychange";                state = "webkitVisibilityState";            }            return {                'hidden': hidden,                'visibilityChange': visibilityChange,                'state': state            };        }            var hidden, s