如何判断传过来的JSON数据中，某个字段是否存在

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

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