var arr = ["lee","hello","zhangsan"];
for(var i in arr) {
  console.log(arr[i]);//lee hello zhangsan
}
数组也是对象 也可以添加自己的属性 我们为arr添加一个name属性
  arr.name = “安妮宝贝”

for(var i in arr) {
  console.log(arr[i]);//lee hello zhangsan 安妮宝贝
}

那么怎么理解该对象来自原型链上的可枚举属性也会被循环？看下面的例子

var bar = {a:1,b:2,c:3};
function foo() {
  this.color = "red";
}
foo.prototype = bar;
var obj = new foo();
for(var prop in obj){
  console.log("o."+prop+"="+obj[prop])//o.color = red   o.a = 1    o.b =2    o.c = 3

}

最后一个例子复制代码运行一下看看结果

<ul id="box">
  <li class="child">1</li>
  <li class="child">2</li>
  <li class="child">3</li>
  <li class="child">4</li>
</ul>
<script>
 var ul = document.querySelector("#box");
 var li = ul.querySelectorAll(".child");
  for(var i in li) {
    console.log(li[i]);
    li[i].onclick = function() {
    alert("hello")
}
}
</script>

         var arr = ["zhangsan","lisi","wangwu"];
         arr.name ="sdfsdfsdf";         arr.forEach(function(element){            console.log(element)// zhangsan lisi  wangwu        })