Page Visibility API 页面是否获取焦点 Event: visibilitychange

W3C 文档 https://www.w3.org/TR/page-visibility/

MDN 文档 https://www.w3.org/TR/page-visibility/

 // Document.visibilityState  可以监听用户是否离开页面，在返回页面
    var t1, {log} = console;

    document.addEventListener("visibilitychange", function() {
            if(this.visibilityState === 'hidden' ){
                t1 = new Date().getTime();
                log('页面对用户不可见.')
            }
            if( this.visibilityState === 'prerender'){
                log('页面此时正在渲染中')
            }
            if( this.visibilityState === 'visible' ){
                log( ~~((Date.now() - t1) / 1000) )
                log('页面内容至少是部分可见')
            }
            if(this.visibilityState === 'unloaded'){
                log('页面从内存中卸载清除')
            }
    });

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    setInterval(()=>{
      console.log( document.hasFocus() );
    }, 200)