locked-array-type.ts

/** Makes a tuple of length `L` with each of the elements of type `T`. */
export type Locked<
  L extends number,
  T,
  $Draft extends unknown[] | readonly unknown[] = []
> =
  // jus making my ternaries look nicer in prettier
  $Draft["length"] extends L
    ? $Draft // ship it
    : $Draft["length"] extends 44 // it will overflow if it's larger than 44 so we need to do the hacky way
    ? T[] & { 0: T; length: L }
    : Locked<L, T, [...$Draft, T]>;

type Foo1 = Locked<2, true>; // [true, true]
    
type Foo2 = Locked<44, string>; // [string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, string, ... 22 more ..., string]

type Foo3 = Locked<45, false | string>; // (false | string)[] & { 0: false | string; length: 45; } <- no overflow!

const foo1: Foo1 = [true, true];

const foo2: Foo2 = ["","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","",""];

const foo3: Foo3 = [...foo2, false]; // `foo2`'s type is explicitly declared so it can compute the expression's type