JS 中的事件绑定、事件监听、事件委托

<input type="button" value="click me" onclick="hello()">

<script>
function hello(){
  alert("hello world!");
}
</script>

<input type="button" value="click me" id="btn">

<script>
document.getElementById("btn").onclick = function(){
  alert("hello world!");
}
</script>

element.addEventListener(event, function, useCapture)

<input type="button" value="click me" id="btn1">

<script>
document.getElementById("btn1").addEventListener("click",hello);
function hello(){
  alert("hello world!");
}
</script>

element.attachEvent(event, function)

<input type="button" value="click me" id="btn2">

<script>
document.getElementById("btn2").attachEvent("onclick",hello);
function hello(){
  alert("hello world!");
}
</script>

<input type="button" value="click me" id="btn3">

<script>
var btn3 = document.getElementById("btn3");
btn3.onclick = function(){
  alert("hello 1"); //不执行
}
btn3.onclick = function(){
  alert("hello 2"); //执行
}
</script>

<input type="button" value="click me" id="btn4">

<script>
var btn4 = document.getElementById("btn4");
btn4.addEventListener("click",hello1);
btn4.addEventListener("click",hello2);

function hello1(){
  alert("hello 1");
}
function hello2(){
  alert("hello 2");
}
</script>

<input type="button" value="click me" id="btn5">

<script>
var btn5 = document.getElementById("btn5");
btn5.addEventListener("click",hello1);//执行了
btn5.addEventListener("click",hello2);//不执行
btn5.removeEventListener("click",hello2);

function hello1(){
  alert("hello 1");
}
function hello2(){
  alert("hello 2");
}
</script>

<input type="button" value="click me" id="btn5">

//绑定监听事件
function addEventHandler(target,type,fn){
  if(target.addEventListener){
    target.addEventListener(type,fn);
  }else{
    target.attachEvent("on"+type,fn);
  }
}

//移除监听事件
function removeEventHandler(target,type,fn){
  if(target.removeEventListener){
    target.removeEventListener(type,fn);
  }else{
    target.detachEvent("on"+type,fn);
  }
}

//测试
var btn5 = document.getElementById("btn5");
addEventHandler(btn5,"click",hello1);//添加事件hello1
addEventHandler(btn5,"click",hello2);//添加事件hello2
removeEventHandler(btn5,"click",hello1);//移除事件hello1

<input type="button" value="click me" id="btn6">

var btn6 = document.getElementById("btn6");
document.onclick = function(event){
  event = event || window.event;
  var target = event.target || event.srcElement;
  if(target == btn6){
    alert(btn5.value);
  }
}

<ul id="list">
  <li id="item1" >item1</li>
  <li id="item2" >item2</li>
  <li id="item3" >item3</li>
</ul>

<script>
var item1 = document.getElementById("item1");
var item2 = document.getElementById("item2");
var item3 = document.getElementById("item3");

item1.onclick = function(){
  alert("hello item1");
}
item2.onclick = function(){
  alert("hello item2");
}
item3.onclick = function(){
  alert("hello item3");
}
</script>

<ul id="list">
  <li id="item1" >item1</li>
  <li id="item2" >item2</li>
  <li id="item3" >item3</li>
</ul>

<script>
var item1 = document.getElementById("item1");
var item2 = document.getElementById("item2");
var item3 = document.getElementById("item3");

document.addEventListener("click",function(event){
  var target = event.target;
  if(target == item1){
    alert("hello item1");
  }else if(target == item2){
    alert("hello item2");
  }else if(target == item3){
    alert("hello item3");
  }
})
</script>

<ul id="list">
  <li id="item1" >item1</li>
  <li id="item2" >item2</li>
  <li id="item3" >item3</li>
</ul>

<script>
var list = document.getElementById("list");

var item = list.getElementsByTagName("li");
for(var i=0;i<item.length;i++){
  (function(i){
    item[i].onclick = function(){
      alert(item[i].innerHTML);
    }
  })(i)
}

var node=document.createElement("li");
var textnode=document.createTextNode("item4");
node.appendChild(textnode);
list.appendChild(node);

</script>