typescript: using enum values in interfaces

Question

I have an interface that uses an enum value in one of its fields:

export enum AnimalType {
    DOG,
    CAT,
}


export interface DogAttrs {
    bones: boolean,
    type: AnimalType.DOG
}

and my goal is a function that creates dogs and adds them to the list of dogs.

function addDog(animalList: DogAttrs[]) {
    const animal = {
        type: AnimalType.DOG,
        bones: true
    }
    animalList.push(animal);   
}

but this function says the object I create is wrong and it does not conform to the DogAttrs interface:

Type 'AnimalType' is not assignable to type '"DOG"'.

why is that? and how to fix this?

live example:

https://www.typescriptlang.org/play/#code/FAUwHgDg9gTgLgAhAOwK4FsEEFkEt0CGANgCoCeEICA3sAvQgCIDyA4ggLwIDkLr3AGjoMAwlhKceYkoOABfYIvDR4CXMjggYAMwIBjKoygBzLHDgwAzjWH0ARlGQhLALgQOoREAWRCGCOAoQN142bnlFRW1UZD04XEcEAgATZKNjAAoffGIAGVxLODd0swtLAG0AXQBKG389R0KkvEIiSVp-f0DKNxwc0iCAOj4-TvtHZzcLVBBbBAV-bNb8wsGIVEsACyyW4mqAbgYIyOAgA

Titian Cernicova-Dragomir · Accepted Answer · 2019-07-18 11:51:45Z

2

The problem is that typescript will widen the type of the constant when you assign them to the variable. The simple solution is to use an as const assertion or to specify the type of the constant explicitly:

function addDog(animalList: DogAttrs[]) {
    const animal: DogAttrs = {
        type: AnimalType.DOG,
        bones: true
    }
    animalList.push(animal);   
}

Or

function addDog(animalList: DogAttrs[]) {
    const animal = {
        type: AnimalType.DOG,
        bones: true
    } as const
    animalList.push(animal);   
}

answered Jul 18, 2019 at 11:51

Titian Cernicova-Dragomir

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