TypeScript tuple inference

tuple.md

You can use the tuple() function in tuple.ts to infer tuple types in TypeScript and cut down on the need to repeat yourself. Without tuple(), declaring a constant of a tuple type looks like this:

const daysOfTheWeek: ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"] = 
  ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"];

You can't do this:

const daysOfTheWeek = ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"]; 
  // right value, wrong type

because TypeScript would infer the type of daysOfTheWeek to be a plain array of string values.

And you can't do this:

const daysOfTheWeek: ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"]; 
  // right type, wrong value

because then the variable daysOfTheWeek would not be defined at runtime.

Enter tuple():

import { tuple } from './tuple'; // or wherever you put your libraries
const daysOfTheWeek = 
  tuple("sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"); // right type, right value

If you inspect the type of daysOfTheWeek, it will be the desired tuple type ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"], and has the correct value at runtime.

---

Note that the inferred tuple type will be as narrow as it can, interpreting any string, number, or boolean literal element as the corresponding literal type:

const narrow = tuple("a", 1, true); // inferred as type ["a", 1, true]

If you want to widen any of them, you can do type assertions:

const wider = tuple("a" as string, 1 as 0 | 1 | 2, true as boolean | undefined) 
// inferred as [string, 0 | 1 | 2, boolean | undefined]

Also note that the library only supports tuples of length up to twelve; you can add more overloads at the top if you need them.

tuple.ts

export type Lit = string | number | boolean | undefined | null | void | {};

// infers a tuple type for up to twelve values (add more here if you need them)
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit, K extends Lit, L extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K, l: L): [A, B, C, D, E, F, G, H, I, J, K, L];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit, K extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K): [A, B, C, D, E, F, G, H, I, J, K];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit, J extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J): [A, B, C, D, E, F, G, H, I, J];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit, I extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I): [A, B, C, D, E, F, G, H, I];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit, H extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H): [A, B, C, D, E, F, G, H];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit, G extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F, g: G): [A, B, C, D, E, F, G];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit, F extends Lit>(a: A, b: B, c: C, d: D, e: E, f: F): [A, B, C, D, E, F];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit, E extends Lit>(a: A, b: B, c: C, d: D, e: E): [A, B, C, D, E];
export function tuple<A extends Lit, B extends Lit, C extends Lit, D extends Lit>(a: A, b: B, c: C, d: D): [A, B, C, D];
export function tuple<A extends Lit, B extends Lit, C extends Lit>(a: A, b: B, c: C): [A, B, C];
export function tuple<A extends Lit, B extends Lit>(a: A, b: B): [A, B];
export function tuple<A extends Lit>(a: A): [A];
export function tuple(...args: any[]): any[] {
  return args;
}

as const is nice!

@laurencestokes I can't reproduce your problem, it works in playground.

as const was perfect. some link to the documentation to know more?

as const was perfect. some link to the documentation to know more?

This feature was added in TS 3.4