javascript - vue gets json data and assigns it to this.data. Why do some key-value pairs appear undefined?

Question

teamData is json obtained from the backend. When using it to make data driver, it always appears,,,,undefined,
Instead, assign those arrays as Empty, and the second-level str is also assigned an empty value, you will not encounter this situation.
How to understand this?

Answer 1

Because it is an Object, when you first load it to get the data, it will definitely be undefined. The official said that it is best to declare all properties in advance. Regardless of whether there is data or not, give him null,

Answer 2

• Make sure you can get json data instead of undefined

• Where did you get the data? Post it and take a look

Answer 3

JSON.parse(res)