TypeScript: extending imported enum

Question

I can merge enum declarations within a single file e.g.

export enum Test {
  value1 = <any>'value1',
  value2 = <any>'value2'
}

export enum Test {
  value3 = <any>'value3'
}

This works fine, but my intention is to have a shared enum which I can extend later, e.g.

// test.enum.ts
export enum Test {
  value1 = <any>'value1',
  value2 = <any>'value2'
}

// place-to-extend-enum.ts
import { Test } from './test.enum';

export enum Test {
  value3 = <any>'value3'
}

What I get is

Individual declarations in merged declaration 'Test' must be all exported or all local.

Is there a way to achieve the desired behaviour?

Chklang · Accepted Answer · 2017-07-10 09:16:13Z

21

In refer to https://github.com/Microsoft/TypeScript/pull/6213 you can do :

// test.enum.ts
export enum Test {
  value1 = <any>'value1',
  value2 = <any>'value2'
}

// place-to-extend-enum.ts
import { Test } from './test.enum';

declare module './test.enum' {
  export enum Test {
    value3 = <any>'value3'
  }
}

... Magic! ;)

answered Jul 10, 2017 at 9:16

Chklang

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4 Comments

smnbbrv Over a year ago

This gives me Error:(25, 14) TS1066:In ambient enum declarations member initializer must be constant expression., so it does not really work with the strings. But I must admit, if I don't assign value3 it does work. +1 though

Chklang Over a year ago

It works with typescript 2.4 which support natively enum as string

smnbbrv Over a year ago

ok, managed to make it work, only v2.4 and still it does not create a new enum. However, it mutates the existing one, so in other words it is still not possible to have another enum... Apart from that, the answer looks nice to me. Thanks!

Jeff Mercado Over a year ago

This doesn't seem to actually work. It extends the interface of the enum so that value3 is considered a valid compile-time value, but the actual runtime value is still undefined.

VJPPaz · Accepted Answer · 2017-07-14 01:34:46Z

I saw a way that you can add additional function/method in an existing enum. this is by create the function within a namespace similar to the enum type: Here

enum Weekday {
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}
namespace Weekday {
    export function isBusinessDay(day: Weekday) {
        switch (day) {
            case Weekday.Saturday:
            case Weekday.Sunday:
                return false;
            default:
                return true;
        }
    }
}

const mon = Weekday.Monday;
const sun = Weekday.Sunday;
console.log(Weekday.isBusinessDay(mon)); // true
console.log(Weekday.isBusinessDay(sun)); // false

You can see the complete information here https://basarat.gitbooks.io/typescript/docs/enums.html at section "Enum with static functions"

Graham · Accepted Answer · 2017-09-22 15:11:24Z

3

After some research I must admit I cannot find a super-proper way to do that.

But there are two possible solutions that are not that bad and not stinking that much.

First is implementing a custom enum - this way is not allowing to consume already existing enums. This is probably the only limitation of this method. Other than that it looks simple and quite native.

Another way is a big hackaround with [merging enums into one value with a separate type declaration. This way allows to consume already existing, real enums; however it is less comfortable to use because there are two entities to be aware of: enum value and enum type.

edited Sep 22, 2017 at 15:11

Graham

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answered Jan 31, 2017 at 8:59

smnbbrv

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