console
function bindEvent (event,type,selector,fn){
console.log(fn)
if(fn==null){
fn=selector;
selector=null
}
var target;
event.addEventListener(type,function(e){
target= e.target;
if(selector){
if(target.matches(selector)){
fn.call(target,e)
}
}else{
fn()
}
})
}
var div1 = document.getElementById('div1');
var btn1 = document.getElementById('btn');
i=4
bindEvent(btn1,'click',function(){
var node=document.createElement("a");
var textnode=document.createTextNode(`a${i++}`);
node.appendChild(textnode);
div1.appendChild(node)
})
bindEvent(div1,'click','a',function(){
alert(this.innerHTML)
})
<div id="div1">
<a href="#">a1</a>
<a href="#">a2</a>
<a href="#">a3</a>
<a href="#">a4</a>
</div>
<button id='btn'>点击增加一个 a 标签</button>