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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