网络异常与网络断开，发送AJAX处理
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
#result {
width: 300px;
height: 100px;
border: 1px solid red;
}
</style>
</head>
<body>
<button>点击发送</button>
</body>
<script>
//当网络存在延迟/异常的特殊处理
const btm = document.getElementsByTagName('button')[0];
let x =null;
let isSending = false;//是否正在发送AJAX请求 true 是 false否
btm.addEventListener('click',function(){
if(isSending){//如果正在发送请求则继续请求，在新建一个请求
x.abort()
};
x= new XMLHttpRequest();
isSending = true;
x.open('GET','http://127.0.0.1:8000/delly');
x.send();
if(x.onreadystatechange === 4){
//修改变量标识
isSending = false;
}
})
</script>
</html>
