Infer a tuple type from an array with values of a known type

April 18, 2023

I am trying to write code that achieves the following:

Imagine I have code elsewhere that defines class Schema<T extends string|number|boolean>,

Now let’s say I write

const superSchema = [
    new Schema<string>(),
    new Schema<number>(),
    new Schema<boolean>(),
    new Schema<number>()
]

I want to define a type called SuperSchemaInfer such that SuperSchemaInfer<typeof superSchema> is equal to [string, number, boolean, number].

How can I do this? I think it should be possible since Zod does something similar.

So far I have written some code, but all I was able to get was SuperSchemaInfer<typeof superSchema> equal to (string|number|boolean)[].

Here is my code so far:

type SupportedTypes = string | number | boolean

type SchemaArray = Array<Schema<SupportedTypes>>

type SchemaArrayInfer<S extends SchemaArray> = {
    [K in keyof S]: S[K] extends Schema<infer U> ? U : never
}

>Solution :

Your problem is not with the type you wrote, but with the type you input

const superSchema = [
    // ^?
    // const superSchema: (Schema<string> | Schema<number> | Schema<boolean>)[]
    new Schema<string>(),
    new Schema<number>(),
    new Schema<boolean>(),
    new Schema<number>()
]

All you need to fix this is to mark array as const and change generic to support ReadonlyArray

type SchemaArrayInfer<S extends readonly Schema<string | number | boolean>[]> = {
    [K in keyof S]: S[K] extends Schema<infer U> ? U : never
}
const superSchema = [
    // ^?
    // const superSchema: (Schema<string> | Schema<number> | Schema<boolean>)[]
    new Schema<string>(),
    new Schema<number>(),
    new Schema<boolean>(),
    new Schema<number>()
] as const
type R = SchemaArrayInfer<typeof superSchema>
//   ^?
// type R = readonly [string, number, boolean, number]