Get type from nested object as type

Question

Is it possible to get only the number keys as an type?

export const messageTypes = {
  ABLEHNUNG_ECON: {
    56: {
      message: "Zählpunkt nicht gefunden",
      event: function () {},
    },
    181: {
      message: "Gemeinschafts-ID nicht vorhanden",
      event: function () {},
    },
  },
  ZUSTIMMUNG_ECON: {
    175: {
      message: "Zustimmung erteilt",
      event: function () {},
    },
  },
  ANTWORT_ECON: {
    99: {
      message: "Meldung erhalten",
      event: function () {},
    },
  },
  ABSCHLUSS_ECON: {
    message: "Abschluss erhalten",
    event: async function (userId: string) {
      await sendNotification(userId, this.message);
    },
  },
};

If i do type MessageT = typeof messageTypes i get the whole object, but is it somehow possible to get only the numbers?

Result should be: type ResponseCodesT = 56 | 181 | 175 | 99

ABSCHLUSS_ECON doesn't have any numeric key inside it, is this intentional or typo? — Cerberus
– Cerberus, Commented Jun 27, 2022 at 11:12
@Cerberus its not typo, its intentional. However, i could technically add an random code in this case, but i first ask if its possible now how it is — Ilijanovic
– Ilijanovic, Commented Jun 27, 2022 at 11:15

Clashsoft · Accepted Answer · 2022-06-27 11:31:01Z

2

It is possible using a little keyof trick (Typescript: keyof typeof union between object and primitive is always never):

export const messageTypes = {
  ABLEHNUNG_ECON: {
    56: {
      message: "Zählpunkt nicht gefunden",
      event: function () {},
    },
    181: {
      message: "Gemeinschafts-ID nicht vorhanden",
      event: function () {},
    },
  },
  ZUSTIMMUNG_ECON: {
    175: {
      message: "Zustimmung erteilt",
      event: function () {},
    },
  },
  ANTWORT_ECON: {
    99: {
      message: "Meldung erhalten",
      event: function () {},
    },
  },
};

type AllUnionMemberKeys<T> = T extends any ? keyof T : never;
type MessageCode = AllUnionMemberKeys<(typeof messageTypes)[keyof typeof messageTypes]>;

// type MessageCode = 56 | 181 | 175 | 99

Playground

I intentionally left out ABSCHLUSS_ECON as an exercise for the reader :) (Hint: the union will include message and event).

answered Jun 27, 2022 at 11:31

Clashsoft

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2 Comments

Matthieu Riegler Over a year ago

Could you explain why is AllUnionMemberKeys required ?

Clashsoft Over a year ago

@MatthieuRiegler see the linked question for details. in short, keyof X | Y produces never if X and Y have no properties in common, while the conditional helps because it distributes the union first and then unions the keys.

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