JavaScript--开关思想

就是男默女泪的立flag!

 <!DOCTYPE html>
 <html lang="en">
 <head>
     <meta charset="UTF-8">
     <title>Title</title>
     <style>
         .video-box{
             width: 1000px;
             height: 500px;
             background-color: skyblue;
             margin: 50px auto;
         }
         body.night{
             background-color: #333;
             color: #fff ;
         }
         body.night button{
             background-color: #ccc;
             color:#fff;
         }

     </style>
     <script>
         window.onload = function () {
             var btn = document.getElementsByTagName('button');
             var flag = false;
             btn[0].onclick = function () {
                 if(!flag) {
                     document.body.className = "night";
                     btn[0].innerHTML = "开灯";
                 }else {
                     document.body.className = "";
                     btn[0].innerHTML = "关灯";
                 }
                 flag = !flag;
             }

 //            var flag = true;
 //            btn[0].onclick = function () {
 //                flag = !flag;
 //                if(!flag) {
 //                    document.body.className = "night";
 //                    btn[0].innerHTML = "开灯";
 //                }else {
 //                    document.body.className = "";
 //                    btn[0].innerHTML = "关灯";
 //                }
 //
 //            }
         }
     </script>
 </head>
 <body>
 <button>关灯</button>
 <h2>页面的文字</h2>
 <div class="video-box"></div>
 </body>
 </html>