如何判斷傳過來的JSON數據中，某個字段是否存在，
1.obj["key"] != undefined
這種有缺陷，如果這個key定義了，並且就是很2的賦值為undefined，那麼這句就會出問題了。
2.!("key" in obj)
3.obj.hasOwnProperty("key")
這兩種方法就比較好了，推薦使用。
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