javascript中用字符串作为下标的数组的遍历

var arr = new Array(5);
arr['a'] = 1;
arr['b'] = 2;
arr['c'] = 3;
arr['d'] = 4;
arr['e'] = 5;
for(var key in arr){
alert(key);
alert(arr[key]);
}
输出结果为：a 1 b 2 c 3 d 4 e 5