js 判断 是否在当前页面

document.addEventListener("visibilitychange", function() {
    console.log(document.visibilityState);
    if(document.visibilityState == "hidden") {
        console.log('隐藏');
    } else if (document.visibilityState == "visible") {
        console.log('显示')
    }
});

var count = 1;
var interval_output = null;
     
function output_number() {
    document.body.innerHTML += (count + " ");
    count++;
}
     
// window 失去焦点，停止输出
window.onblur = function() {
    clearInterval(interval_output);
};
     
// window 每次获得焦点
window.onfocus = function() {
    // 每 1 秒在页面输出一个数
    interval_output = setInterval(function() {
        output_number();
    }, 1000);
}