Describe interface Fixed values in Array element of TypeScript

Question

I am trying to use TypeScript to describe the interface for an object. Faced such a problem: How to set an opportunity with help of an interface, limited values in an array. Example:

export interface IChannel {
  title: string
  name: string,
  signal: ["discrete", "analog", "text"] | ["discrete", "analog"] | ["discrete"] | ["analog"] | ["text"] | ["analog", "text"], // OMG... IT"S BAD !!!
  enabled: boolean
  "description": string,
  "isChangeVisibility": boolean
}

Need filed signal = ["discrete" OR/AND "analog" OR/AND "text"] This field must contain only these values. Array length (may be 1, 2 or 3 elements) Thanks for any help. I will sit experimenting.

paolostyle · Accepted Answer · 2020-05-19 11:23:56Z

3

Using an enum like @axiac suggested is one solution, but then if you want to create an object of type IChannel you have to use the enum values (e.g. you can't do signal: ['discrete'], it has to be signal: [SignalType.discrete] or signal: ['discrete'] as SignalType[]).

IMO an union of literal types is simpler and does not produce any additional JavaScript output (enums are compiled to objects):

type SignalType = 'discrete' | 'analog' | 'text';

export interface IChannel {
  title: string;
  name: string;
  signal: SignalType[];
  enabled: boolean;
  description: string;
  isChangeVisibility: boolean;
}

The downside is that something like ['discrete', 'discrete'] will be okay for the compiler. If it's important to enforce uniqueness of values then your current solution is the only reasonable one I think.

You could also do signal: [SignalType, SignalType?, SignalType?], which enforces the number of elements to 1, 2 or 3 (but still doesn't solve the issue with value uniqueness).

edited May 19, 2020 at 11:23

answered May 17, 2020 at 13:36

paolostyle

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1 Comment

Vad0k Over a year ago

Thank you @paolostyle. This works well. At the expense of uniqueness is not critical. It is ideal for a compiler.

axiac · Accepted Answer · 2020-05-17 11:48:57Z

1

An enum is the best type for the values that can be used in the field signal:

enum SignalType {
  discrete = 'discrete',
  analog = 'analog',
  text = 'text',
}

export interface IChannel {
  title: string
  name: string,
  signal: SignalType[]
  enabled: boolean
  description: string,
  isChangeVisibility: boolean
}

answered May 17, 2020 at 11:48

axiac

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3 Comments

VRoxa Over a year ago

But this does not constraint the possibility of duplicate keys (?..

paolostyle Over a year ago

@VRoxa it doesn't, but there's no good way to achieve it any other way than what OP already has, stackoverflow.com/a/57021889/1708094 could help but IMO it's not worth the effort

axiac Over a year ago

Another option is to make signal a numeric field and use numeric values combined with the bitwise OR operator. This has advantages (the duplicate values in its definition do not matter) and disadvantages (the type accepts invalid values, the operations are a bit more cryptic for newbies).

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