<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
ul{
list-style: none;
}
#traffic>li{
display:block;
}
#traffic span{
display:inline-block;
50px;
height:50px;
background-color:gray;
margin:5px;
border-radius:50%;
float:left;
}
#traffic.stop li:nth-child(1) span{
background-color:yellow;
}
#traffic.wait li:nth-child(2) span{
background-color:red;
}
#traffic.pass li:nth-child(3) span{
background-color:green;
}
</style>
</head>
<body>
<ul id="traffic">
<li><span></span></li>
<li><span></span></li>
<li><span></span></li>
</ul>
<script>
/*const traffic = document.getElementById("traffic");
(function reset(){
traffic.className = "wait";
setTimeout(function(){
traffic.className = "stop";
setTimeout(function(){
traffic.className = "pass";
setTimeout(reset,2000);
},2000);
},2000);
})();*/
const traffic = document.getElementById("traffic");
var stateList = ["wait", "stop", "pass"];
var currentStateIndex = 0;
setInterval(function(){
var state = stateList[currentStateIndex];
traffic.className = state;
currentStateIndex = (currentStateIndex + 1) % stateList.length;
},2000);
</script>
</body>
</html>