In JS, how does one determine whether a field exists in the transmitted JSON data?

In JS, how does one determine whether a field exists in the transmitted JSON data?

How to determine whether a field exists in the transmitted JSON data,

1. obj ["key"]! = Undefined

This is flawed. If the key is defined and the value of 2 is undefined, a problem will occur.

2 .! ("Key" in obj)
3. obj. hasOwnProperty ("key ")

The two methods are better. We recommend that you use them.

Original answer:

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 shoshould 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 maid test for properties of the object instance (and not inherited properties), usehasOwnProperty:

Obj. hasOwnProperty ("key") // true

How can I determine whether a field exists when jsoncpp reads a json file?

If (value ["sex"]. isNull ()... is to check whether a key name exists in the array.

 
After Jquery flexigird obtains json data, it cannot be determined on the foreground. If a field is the same, do some operations?

Json is sent to the front-end. It looks like it's playing. I don't know why it's called json. But js is javascript, your task is to export the json to the foreground.
2. Find the field you want, for example, mJson ['name'] = "zhangsan" (in one dimension)
MyJSONObject. arr1 [0]. name = "Zhang San" (two-dimensional)
Are there any other things that you can't do?