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