如何判断传过来的json数据中，某个字段是否存在，
1.obj[key] != undefined
这种有缺陷，如果这个key定义了，并且就是很2的赋值为undefined，那么这句就会出问题了。
2.!(key in obj)
3.obj.hasownproperty(key)
这两种方法就比较好了，推荐使用。
答案原文：
actually, checking for undefined-ness is not an accurate way of testing whether a key exists. what if the key exists but the value is actually undefined?
var obj = { key: undefined };
obj[key] != undefined // false, but the key exists!
you should instead use the in operator:
key in obj // true, regardless of the actual value
if you want to check if a key doesn't exist, remember to use parenthesis:
!(key in obj) // true if key doesn't exist in object
!key in obj // error! equivalent to false in obj
or, if you want to particularly test for properties of the object instance (and not inherited properties), usehasownproperty:
obj.hasownproperty(key) // true