js 中的[] {}是什么意思

<div id="aa1">aaaaa</div>
<div id="aa2">bbbb</div>
<div id="aa3">cccc</div>
<script>
//用{}定义一组对像
var obj = {'a':1111,'b':2222,'c':'abc','d':function(){
alert('对像：d函数');
}}; 
alert('对像：'+obj.b);//调用对像b
obj.d(); //调用对像d函数

//定义一数组用[]设置索引值
var arr = new Array();
arr[0] = 1111;
arr[1] = 2222;
arr[2] = '数组：abcdefg';

alert(arr[2]); //调用数组键名索引为2的值。

//用[]定义数组
var arr2=['pppp','opopop'];
alert(arr2[1]);

//中括号还有一个用法!
for(var i=1;i<=3;i++){
	alert('div：'+document.getElementById(['aa'+i]).innerHTML);
}

//JSON中同时使用[]和{}
var json = [{"arr1":{"a":123,"b":456},"arr2":{"a":789,"c":"abcdefg"}},{"arr1":{"a":"uuuuu","b":"oooo"},"arr2":{"a":"aaa9999","c":"abcdefg"}}];
alert('JSON1:'+json[0].arr2.a);
alert('JSON2:'+json[1].arr2.a);
</script>