<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<button class="btn">按钮</button>
<script>
var i = 0;
var btn=document.querySelector(".btn");
function on(ele,eventStr,callback){
if(ele.addEventListener){
ele.addEventListener(eventStr,callback);
}else{
ele.attachEvent("on"+event,callback)
}
}
function off(ele,eventStr,callback,n){
if(ele.removeEventListener){
ele.removeEventListener(eventStr,callback)
}else{
ele.detachEvent("on"+eventStr,callback)
}
}
function func(){
alert(i);
i++;
if(i>2){
off(btn,"click",func)
}
}
on(btn,"click",func)
</script>
</body>
</html>