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