42 lines
1.5 KiB
TypeScript
42 lines
1.5 KiB
TypeScript
/**
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Creates a read-only tuple of type `Element` and with the length of `Length`.
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@private
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@see `ReadonlyTuple` which is safer because it tests if `Length` is a specific finite number.
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*/
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type BuildTupleHelper<Element, Length extends number, Rest extends Element[]> =
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Rest['length'] extends Length ?
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readonly [...Rest] : // Terminate with readonly array (aka tuple)
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BuildTupleHelper<Element, Length, [Element, ...Rest]>;
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/**
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Create a type that represents a read-only tuple of the given type and length.
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Use-cases:
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- Declaring fixed-length tuples with a large number of items.
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- 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.
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- Creating a tuple of coordinates with a static length, for example, length of 3 for a 3D vector.
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@example
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```
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import {ReadonlyTuple} from 'type-fest';
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type FencingTeam = ReadonlyTuple<string, 3>;
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const guestFencingTeam: FencingTeam = ['Josh', 'Michael', 'Robert'];
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const homeFencingTeam: FencingTeam = ['George', 'John'];
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//=> error TS2322: Type string[] is not assignable to type 'FencingTeam'
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guestFencingTeam.push('Sam');
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//=> error TS2339: Property 'push' does not exist on type 'FencingTeam'
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```
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@category Utilities
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*/
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export type ReadonlyTuple<Element, Length extends number> =
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number extends Length
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// Because `Length extends number` and `number extends Length`, then `Length` is not a specific finite number.
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? readonly Element[] // It's not fixed length.
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: BuildTupleHelper<Element, Length, []>; // Otherwise it is a fixed length tuple.
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