数组下标

数组下标可以为任意表达式，甚至为任意数据类型：

1、数字作为下标

2、字符串作为下标

1）数组本身也是对象，这些字符串会被结实为对象的属性，数组本身的length不会发生变化

2）与对象的类同结构

var arr1 = new Array();
arr1['name'] = "xhaja";
arr1['age'] = 34
for(var key in arr1) {
   console.log(key)         //name , age
   console.log(arr1[key])   // xhaja , 34
}
console.log(arr1.length) // 结果为0

var arr = []
arr["1"] = 100;
console.log(arr["1"])  //100
console.log(arr[1])    //100
        
arr[true] = 100;
console.log(arr[true]) //100
arr[-1] = 100;
console.log(arr["-1"]) //100
console.log(arr[-1])   //100

应用：哈希表