var btn = document.querySelector("#btn");
btn.onclick=function () {
 console.log(event.currentTarget === this); //true
 console.log(event.target === this); //true
}

var btn = document.querySelector("#btn");
document.body.onclick=function () {
 console.log(event.currentTarget === document.body); //true
 console.log(this === document.body); //true
 console.log(event.target === btn); //true 因为btn没有注册事件处理程序，所以该click事件就冒泡到了document.body
}

//获取按钮
var btn = document.querySelector("#btn");
//设置多个事件
var handler = function() {
//检测事件的类型
 switch (event.type) {
  case "click":
   console.log("i click it");
   break;
  case "mouseover":
   console.log("i enter it");
   break;
  case "mouseout":
   console.log("i leave it");
   break;
 }
}
//给响应的事件赋值
btn.onclick = handler;
btn.onmouseover = handler;
btn.onmouseout = handler;
preventDefault()

var aTags = document.getElementsByTagName("a");
for (var i = 0; i < aTags.length; i++) {
 var currentATag = aTags[i];
 currentATag.onclick = function() {
  event.preventDefault();
 }
};

var btn = document.getElementById("btn");
btn.onclick = function () {
 console.log("btn clicked");
 // event.stopPropagation();
};
window.onclick = function () {
 console.log("clicked");
};
//单击一下的结果：
//btn clicked
//clicked

var btn = document.getElementById("btn");
btn.onclick = function () {
 console.log("btn clicked");
 event.stopPropagation();
};
window.onclick = function () {
 console.log("clicked");
};
//单击一下的结果：
//btn clicked

var btn = document.getElementById("btn");

document.body.addEventListener("click", function() {
 console.log("bodyListener" + event.eventPhase);
}, true) //捕获阶段

btn.onclick = function() {
 console.log("btn" + event.eventPhase);
} //目标对象阶段，实际上属于冒泡阶段（在btn上）

document.body.onclick = function() {
 console.log("body" + event.eventPhase);
} //冒泡阶段（在body上）

var btn = document.getElementById("btn");

document.body.addEventListener("click", function() {
 console.log(event.eventPhase); //1
 console.log(event.currentTarget); //HTMLBodyElement
}, true);

btn.addEventListener("click", function() {
 console.log(event.eventPhase); //2
 console.log(event.currentTarget); //HTMLInputElement
});

document.body.addEventListener("click", function() {
 console.log(event.eventPhase); //3
 console.log(event.currentTarget); //HTMLBodyElement
});