callback.call(context || null ,obj[i],arg)
context 是 this obj[i] 是数组的元素
arg 是什么？ 有什么作用啊
arg是传入的参数，就是‘aaa’
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script src="../jquery-2.1.0.js" type="text/javascript"></script>
<title></title>
</head>
<body>
<table border="1">
<tr><td>TD #0</td><td>TD #1</td><td>TD #2</td></tr>
<tr><td>TD #3</td><td>TD #4</td><td>TD #5</td></tr>
<tr><td>TD #6</td><td>TD #7</td><td>TD #8</td></tr>
</table>
<ul>
<li>list item 1</li>
<li>list item 2</li>
<li>list item 3</li>
<li>list item 4</li>
<li>list item 5</li>
</ul>
<!-- <script type="text/javascript">
$("td:eq(2)").css("color", "red")
$('li').eq(2).css('background-color', 'red');
$('li').eq(-2).css('background-color', 'pink');
</script> -->
<script type="text/javascript">
function each(obj, callback, context, arg) {
var i = 0;
var value;
var length = obj.length;
for (; i < length; i++) {
arg = i;
callback.call(context || null, obj[i], arg);
}
}
var arr = ['a', 'b', 'c'];
each(arr, function(name, arg) {
console.log(arg) //相当于索引
$('body').append('<li>' + name + '</li>')
}, this, 'aaa')
</script>
</body>
</html>