javascript中in运算符用法分析

var point = {x:1,y:1};
"x" in point //true
"z" in point //false
"toString" in point //true
var ary = [1,2,3];
"0" in ary; //true，ary含有索引0("0"回转换为0)；
1 in ary;  //true,ary含有索引1
3 in ary;  //false