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{}定義一組對像
var obj = {'a':1111,'b':2222,'c':'abc','d':function(){
alert('對像：d函數');
}};
alert('對像：'+obj.b);//調用對像b
obj.d(); //調用對像d函數

//定義一數組用[]設置索引值
var arr = new Array();
arr[0] = 1111;
arr[1] = 2222;
arr[2] = '數組：abcdefg';

alert(arr[2]); //調用數組鍵名索引為2的值。

//用[]定義數組
var arr2=['pppp','opopop'];
alert(arr2[1]);

//中括號還有一個用法!
for(var i=1;i<=3;i++){
alert('div：'+document.getElementById(['aa'+i]).innerHTML);
}

//JSON中同時使用[]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>