How to extend an existing observable with a getter computed

[stellarhoof]

I'm able to use this pattern in mobx 4

let m = require('mobx')

let a = m.observable({ a: 1 }, {})

m.extendObservable(a, {
  get hidden() {
    return true
  },
}).hidden

However, it gives me this error when migrating to mobx6

Uncaught Error: [MobX] Cannot apply 'true' to '[email protected]':
The field is already annotated with 'true'.
Re-annotating fields is not allowed.
Use 'override' annotation for methods overriden by subclass.
    at die (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:91:11)
    at assertAnnotable (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:5994:5)
    at ObservableObjectAdministration.extend_ (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:5530:5)
    at /Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:3056:11
    at Array.forEach (<anonymous>)
    at Object.extendObservable (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:3055:26)

What's the idiomatic way of extending an observable with a getter?

Thanks!

[mweststrate]

Can you provide a minimal reproduction?

…

On Wed, 12 Oct 2022, 21:55 Alejandro Hernandez, ***@***.***> wrote: I'm able to use this pattern in mobx 4 let m = require('mobx') let a = m.observable({ a: 1 }, {}) m.extendObservable(a, { get hidden() { return true }, }) However, it gives me this error when migrating to mobx6 Uncaught Error: [MobX] Cannot apply 'true' to ***@***.***': The field is already annotated with 'true'. Re-annotating fields is not allowed. Use 'override' annotation for methods overriden by subclass. at die (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:91:11) at assertAnnotable (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:5994:5) at ObservableObjectAdministration.extend_ (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:5530:5) at /Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:3056:11 at Array.forEach (<anonymous>) at Object.extendObservable (/Users/ah/Code/smartprocure/spark/node_modules/mobx/dist/mobx.cjs.development.js:3055:26) — Reply to this email directly, view it on GitHub <#3541>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAN4NBDMNP5BSBLL4AGPMULWC4JRRANCNFSM6AAAAAARDTN744> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[stellarhoof]

The fenced code above reproduces the behavior I'm seeing

[urugator]

Can't reproduce https://codesandbox.io/s/extendobservable-todo-del-u13w5p