type-fest/source/readonly-tuple.d.ts

/**
Creates a read-only tuple of type `Element` and with the length of `Length`.

@private
@see `ReadonlyTuple` which is safer because it tests if `Length` is a specific finite number.
*/
type BuildTupleHelper<Element, Length extends number, Rest extends Element[]> =
	Rest['length'] extends Length ?
		readonly [...Rest] : // Terminate with readonly array (aka tuple)
		BuildTupleHelper<Element, Length, [Element, ...Rest]>;

/**
Create a type that represents a read-only tuple of the given type and length.

Use-cases:
- Declaring fixed-length tuples with a large number of items.
- Creating a range union (for example, `0 | 1 | 2 | 3 | 4` from the keys of such a type) without having to resort to recursive types.
- Creating a tuple of coordinates with a static length, for example, length of 3 for a 3D vector.

@example
```
import {ReadonlyTuple} from 'type-fest';

type FencingTeam = ReadonlyTuple<string, 3>;

const guestFencingTeam: FencingTeam = ['Josh', 'Michael', 'Robert'];

const homeFencingTeam: FencingTeam = ['George', 'John'];
//=> error TS2322: Type string[] is not assignable to type 'FencingTeam'

guestFencingTeam.push('Sam');
//=> error TS2339: Property 'push' does not exist on type 'FencingTeam'
```

@category Utilities
*/
export type ReadonlyTuple<Element, Length extends number> =
	number extends Length
		// Because `Length extends number` and `number extends Length`, then `Length` is not a specific finite number.
		? readonly Element[] // It's not fixed length.
		: BuildTupleHelper<Element, Length, []>; // Otherwise it is a fixed length tuple.