How to extract tuple type based on prefix using TypeScript?

Question

I've gotten pretty close already.

I have a set of tuples:

type Schema =
    | ["link", string, string]
    | ["recentFiles", "byPath", string, number]
    | ["recentFiles", "byTime", number, string]

I created this type to iterate through all the prefixes:

type TuplePrefix<T extends unknown[]> = T extends [any, ...infer U]
    ? [] | [T[0]] | [T[0], ...TuplePrefix<U>]
    : []


type Prefixes = TuplePrefix<Schema>

And now I want to filter this list based on whether it is prefixed by another tuple.

type Result1 = Extract<Schema, ["recentFiles"]> // ❌ never
type Result2 = Extract<Schema, {0: "recentFiles"}> // ✅ ["recentFiles", "byPath", string, number] | ["recentFiles", "byTime", number, string]

So it appears this extraction doesn't work for tuples.

I tried mapping from tuple to the object representation, but TypeScript won't have it.

type TupleToObject<T extends any[]> = {[K in keyof T]: T[K]}

Any ideas how to make this work?

playground

Chet · Accepted Answer · 2022-03-13 09:58:43Z

2

A little bit of hardcoding, but this actually works:

type Ints = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "10" | "11" | "12" | "13" | "14" | "15" // | `${Ints}${Ints}`
type TupleToObject<T extends any[]> = Pick<T, Extract<keyof T, Ints>>

type Result = Extract<Schema, TupleToObject<["recentFiles"]>>

answered Mar 13, 2022 at 9:58

Chet

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1 Comment

Nicolò Over a year ago

To avoid hard-coding, you can now use type TupleToObject<T extends any[]> = Pick<T, keyof T & `${number}`>

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