Defining array of enum type in Typescript

Question

I have created a Typescript DTO, where I want to ensure type safety for a list of strings (which are derived from an enum type.



export class ConnectedUserWithPhotosDTO extends UserWithPhotosDTO {
  userTags: keyof User["userTags"][]

  constructor(user: User, photos: ResponsePhotoDTO[]) {
    super(user, photos)
    console.log(user.userTags)
    this.userTags = user.userTags;
  }
}

The typescript compiler complains about the following error:

Type 'UserTags[]' is not assignable to type 'number | keyof UserTags[][]'.

apparently, the value of userTags evaluates to

number | keyof UserTags[][]

Here is the enum definition that i am defining.

export enum UserTags {
  FITNESS = 'Fitness',
  FOURTWENTY_FRIENDLY = '420 Friendly',
  MEDITATION = 'Meditation',
  DRINKS = 'Drinks',
  DOGS = 'Dogs',
  CATS = 'Cats',
  FASHION = 'Fashion',
  WINE_TASTING = 'Wine Tasting',
  FOODIE = 'Foodie',
  ART = 'Art',
  PARTYING = 'Partying',
  TRAVELIING = 'Travelling',
  GAMING = 'Gaming',
}

Inside the User class the UserTags is defined as:

 @ApiProperty({ enum: UserTags, isArray: true, default: [] })
  @Column('enum', { enum: UserTags, array: true, nullable: true, default: [] })
  userTags: UserTags[]

how can I define just the type of keyof for the specific Enum value?

Provide the User type. It's not clear what you are trying to do. — Andrei Tătar
– Andrei Tătar, Commented Mar 16, 2021 at 18:17
I'm 99% sure that you just want the string enums and want userTags: UserTags[], but I'll explain in an answer — Linda Paiste
– Linda Paiste, Commented Mar 16, 2021 at 18:45

Timothy · Accepted Answer · 2021-03-16 18:18:38Z

7

Assuming that UserTags is enum. Its something like

userTags: Array<keyof typeof UserTags>

answered Mar 16, 2021 at 18:18

Timothy

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3 Comments

Kristoffer Tølbøll Over a year ago

Problem is that UserTags is a nested attribute on the User Class

Timothy Over a year ago

Provide user class definition, please

Linda Paiste Over a year ago

Your answer is what they asked for - the keys of the enum object. But I don't think it's what they actually want. They are obviously confused, but I think they want the enum values Array<UserTags>.

Andrei Tătar · Accepted Answer · 2021-03-16 18:30:40Z

2

You can also do something like:

const userTags: User['userTags'];

answered Mar 16, 2021 at 18:30

Andrei Tătar

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2 Comments

Kristoffer Tølbøll Over a year ago

How does the compiler know if it a single instance or a list?

Andrei Tătar Over a year ago

@Ating it just uses the exact same type of the userTags property in the User type.

Linda Paiste · Accepted Answer · 2021-03-16 19:00:56Z

Your post is a bit confusing because of your use of keyof. It's unclear whether you want the enum keys "FITNESS", etc. or the enum values "Fitness", etc.

It's clear based on the code of the User and your error message that what you are getting when you access user.userTags is the values. The type of the values is just the enum itself, so you would want

userTags: UserTags[]

just like what you have in the User. It makes sense that this would match the type in User since we are setting the userTags property based on User["userTags"].

Let's break down the type keyof User["userTags"][] that you tried and see what it really means.

User["userTags"] gives us the userTags property of the User type. We can see from your User code that User["userTags"] is UserTags[]. That is an array of the values from the UserTags enum. So something like ["Fitness", "Fashion"].

User["userTags"][] means that we want an array of these. But it's already an array, so you are now asking for an array of arrays of enum values: UserTags[][]. Which would be [["Fitness"], ["Fashion", "Foodie"]].

keyof User["userTags"][] means that we now want the keys of that array of arrays: keyof UserTags[][]. The key of an array is a number so typescript gives you number | keyof UserTags[][]. (I'm not sure why it isn't just number).

Perhaps you were trying to get the keys for the UserTags enum object? keyof typeof UserTags will give you the union type "FITNESS" | "FOURTWENTY_FRIENDLY" | "MEDITATION" .... But it doesn't seem like that's what you actually want to use. You want the union of the enum values which is just UserTags.

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