There is a little pit. Note that you cannot use console.log on root instantiation. If you open vconsole, an error will be reported! ! !

      1 Open vconsole and report toJSON is defiend. The positioning problem is in the console.log() and it will not report an error.

      2 If you need to output this thing, please find a place to define it toJSON(){} Although there is no error, I don’t know if there is any problem.

      3 Do not open vconsole without this bug

      4 Comment out when not needed

The reason for the problem is explained on the official website as Vue does not allow dynamic addition of root-level responsive attributes, so you must initialize the Vue instance by pre-declaring all root-level responsive data attributes, even if the value is empty:

Official website link  https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties