作用：
遍历对象的属性
语法：
for (variable in object){}
实例：
var obj = {a:1, b:2, c:3};for(key in obj){ console.log(key)//a,b,c console.log(obj[key])//1,2,3}遍历对象的属性
for (variable in object){}
var obj = {a:1, b:2, c:3};for(key in obj){ console.log(key)//a,b,c console.log(obj[key])//1,2,3}