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ExcludeRestElement: Fix generic assignability with arrays #1274

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Merged
sindresorhus merged 4 commits into from
Oct 25, 2025
+37 −6
Merged

ExcludeRestElement: Fix generic assignability with arrays #1274

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8bba628
fix: generic assignability
som-sm Oct 17, 2025
dc490d4
test: add generic assignability case of SplitOnRestElement
som-sm Oct 17, 2025
0aed831
test: add cases for long tuples
som-sm Oct 17, 2025
41a905f
test: add cases for long tuples for SplitOnRestElement & ExtractRestE…
som-sm Oct 17, 2025
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8 changes: 5 additions & 3 deletions source/exclude-rest-element.d.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
import type {SplitOnRestElement} from './split-on-rest-element.d.ts';
import type {IsArrayReadonly} from './internal/array.d.ts';
import type {UnknownArray} from './unknown-array.d.ts';
import type {IfNotAnyOrNever} from './internal/type.d.ts';

/**
Create a tuple with the [`rest`](https://www.typescriptlang.org/docs/handbook/2/objects.html#tuple-types) element removed.
Expand All @@ -26,13 +27,14 @@ type T4 = ExcludeRestElement<[number, string]>;
@see {@link SplitOnRestElement}
@category Array
*/
export type ExcludeRestElement<Array_ extends UnknownArray> =
export type ExcludeRestElement<Array_ extends UnknownArray> = IfNotAnyOrNever<Array_,
SplitOnRestElement<Array_> extends infer Result
? Result extends readonly UnknownArray[]
? IsArrayReadonly<Array_> extends true
? Readonly<[...Result[0], ...Result[2]]>
: [...Result[0], ...Result[2]]
: Result
: never;
: never
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@som-sm som-sm Oct 17, 2025

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If this is Result (and not never), then the compiler thinks the type produces something more than just arrays.

: never
>;
Comment on lines +30 to +38
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@benzaria benzaria Oct 17, 2025 •
edited
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Please consider using IsAnyOrNever instead, as it provides more reliable handling for recursive types.
Refer my #1192 comment, and your #1266 comment regarding If* types.

Suggested change
export type ExcludeRestElement<Array_ extends UnknownArray> = IfNotAnyOrNever<Array_,
SplitOnRestElement<Array_> extends infer Result
? Result extends readonly UnknownArray[]
? IsArrayReadonly<Array_> extends true
? Readonly<[...Result[0], ...Result[2]]>
: [...Result[0], ...Result[2]]
: Result
: never;
: never
: never
>;
export type ExcludeRestElement<Array_ extends UnknownArray> =
IsAnyOrNever<Array_> extends true ? Array_
: SplitOnRestElement<Array_> extends infer Result
? Result extends readonly UnknownArray[]
? IsArrayReadonly<Array_> extends true
? Readonly<[...Result[0], ...Result[2]]>
: [...Result[0], ...Result[2]]
: never
: never;

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@som-sm som-sm Oct 17, 2025 •
edited
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IfNotAnyOrNever works perfectly fine in this case because ExcludeRestElement is not a recursive type. Added test cases for long tuples in #0aed831.


your #1266 comment regarding If* types.

This was only for recursive types.

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@som-sm som-sm Oct 17, 2025

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And, if used properly, IfNotAnyOrNever can even be used with recursive types without any problems. Added a note in #1276.


export {};
6 changes: 5 additions & 1 deletion test-d/exclude-rest-element.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import {expectType} from 'tsd';
import type {ExcludeRestElement} from '../index.d.ts';
import type {ExcludeRestElement, UnknownArray} from '../index.d.ts';

// Basic static tuples (No rest element)
expectType<ExcludeRestElement<[]>>({} as []);
Expand Down Expand Up @@ -51,3 +51,7 @@ expectType<ExcludeRestElement<[['a'], ...string[], ['z']]>>({} as [['a'], ['z']]
// Edge: `never` / `any`
expectType<ExcludeRestElement<any>>({} as any);
expectType<ExcludeRestElement<never>>({} as never);

// Generic instantiations
type Assignability<_T extends UnknownArray> = unknown;
type TestAssignability<T extends UnknownArray> = Assignability<ExcludeRestElement<T>>;
6 changes: 5 additions & 1 deletion test-d/split-on-rest-element.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import {expectType} from 'tsd';
import type {SplitOnRestElement} from '../index.d.ts';
import type {SplitOnRestElement, UnknownArray} from '../index.d.ts';

// Fixed tuples (No rest element)
expectType<SplitOnRestElement<[]>>({} as [[], [], []]);
Expand Down Expand Up @@ -53,3 +53,7 @@ expectType<SplitOnRestElement<readonly [1, 2, 3]>>({} as readonly [[1, 2, 3], []
// Edge: `never` / `any`
expectType<SplitOnRestElement<any>>({} as any);
expectType<SplitOnRestElement<never>>({} as never);

// Generic instantiations
type Assignability<_T extends UnknownArray> = unknown;
type TestAssignability<T extends UnknownArray> = Assignability<SplitOnRestElement<T>>;