判断当前页面是不是用户正在浏览的页面

通过visibilitychange事件来判断页面可见性的状态，通过document.hidden来判断当前页面是否在浏览

// 各种浏览器兼容
var hidden, visibilityChange; 
if (typeof document.hidden !== "undefined") {
    hidden = "hidden";
    visibilityChange = "visibilitychange";
} else if (typeof document.mozHidden !== "undefined") {
    hidden = "mozHidden";
    visibilityChange = "mozvisibilitychange";
} else if (typeof document.msHidden !== "undefined") {
    hidden = "msHidden";
    visibilityChange = "msvisibilitychange";
} else if (typeof document.webkitHidden !== "undefined") {
    hidden = "webkitHidden";
    visibilityChange = "webkitvisibilitychange";
}

// 添加监听器，在title里显示状态变化
document.addEventListener(visibilityChange, function() {
    if(document[hidden]){
        // 用户没有浏览当前页面
    }else{
        // 用户正在浏览当前页面
    }
}, false);

　　或者

var hiddenProperty = 'hidden' in document ? 'hidden' :    
    'webkitHidden' in document ? 'webkitHidden' :    
    'mozHidden' in document ? 'mozHidden' :    
    null;
var visibilityChangeEvent = hiddenProperty.replace(/hidden/i, 'visibilitychange');
var onVisibilityChange = function(){
    if (!document[hiddenProperty]) {    
        console.log('页面非激活');
    }else{
        console.log('页面激活')
    }
}
document.addEventListener(visibilityChangeEvent, onVisibilityChange);