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Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
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/** | ||
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]>; | ||
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/** | ||
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. |
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import {expectAssignable, expectError, expectNotAssignable} from 'tsd'; | ||
import {ReadonlyTuple} from '../index'; | ||
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type TupleOfThreeStrings = ReadonlyTuple<string, 3>; | ||
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expectAssignable<TupleOfThreeStrings>(['a', 'b', 'c']); | ||
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expectNotAssignable<TupleOfThreeStrings>(['a', 'b', 123]); | ||
expectNotAssignable<TupleOfThreeStrings>(['a']); | ||
expectNotAssignable<TupleOfThreeStrings>(['a', 'b']); | ||
expectNotAssignable<TupleOfThreeStrings>(['a', 'b', 'c', 'd']); | ||
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declare const test: TupleOfThreeStrings; | ||
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expectError(test.push); | ||
expectError(test[2] = 'a'); |