TypeScript and Express create a powerful array combination capability within the type system

Question

TypeScript and Express create a powerful array combination capability within the type system

After defining the EventType as either "TYPE A" or "TYPE B", I am looking to create a type for an array that can only contain one or both of these event types.

Simply typing it as an EventType[] allows duplicates, which is not ideal.

type Test = EventType[]

const eventTypes:Test = ["TYPE A", "TYPE B", "TYPE B", "TYPE A"]

Listing out all available options individually becomes cumbersome when there are more than two options:

type Test = ["TYPE A"] | ["TYPE B"] | ["TYPE A", "TYPE B"];

// This will throw a compiler error
const eventTypes:Test = ["TYPE A", "TYPE B", "TYPE B", "TYPE A"]

Is there a way to achieve this constraint at the type level in TypeScript without explicitly enumerating every option?

typescript

Answer 1

Answer №1

Here is an example that might be useful

const eventTypes = ["TYPE X", "TYPE Y"] as const
type Example = [typeof eventTypes[number]] | typeof eventTypes

const onlyX: Example = ["TYPE X"]
const onlyY: Example = ["TYPE Y"]
const allTypes: Example = ["TYPE X", "TYPE Y"]

// encountering compiler errors
const repeatedType: Example = ["TYPE X", "TYPE X"]
const emptyValue: Example = []

Playground link

Answer 2

Here is an example that might be useful

const eventTypes = ["TYPE X", "TYPE Y"] as const
type Example = [typeof eventTypes[number]] | typeof eventTypes

const onlyX: Example = ["TYPE X"]
const onlyY: Example = ["TYPE Y"]
const allTypes: Example = ["TYPE X", "TYPE Y"]

// encountering compiler errors
const repeatedType: Example = ["TYPE X", "TYPE X"]
const emptyValue: Example = []

Playground link

TypeScript and Express create a powerful array combination capability within the type system

Answer №1

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