E533 `MergeDeep` : fix producing `[never]` when merging empty tuples. by taiyakihitotsu · Pull Request #1330 · sindresorhus/type-fest · GitHub
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MergeDeep : fix producing [never] when merging empty tuples. #1330

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MergeDeep : fix producing [never] when merging empty tuples. #1330

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@taiyakihitotsu taiyakihitotsu commented Jan 23, 2026 •
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Close #1200

Test case

I use IsEqual because this passes uncorrectly on editor (it rejects correctly with npm test though).

expectType<{f: [0]}>({} as {f: [never]})

I think this might be related to this tsd issue

Undefined behavior

type A = MergeDeep<[1, 2], string[], {recurseIntoArrays: true, arrayMergeMode: 'replace'}>
// [string, string, ...string[]]
type B = MergeDeep<[1, 2], string[], {recurseIntoArrays: true, arrayMergeMode: 'spread'}> 
// [string, string, ...string[]]
type C = MergeDeep<string[], [1, 2], {recurseIntoArrays: true, arrayMergeMode: 'replace'}> 
// [1, 2, ...string[]]
type D = MergeDeep<string[], [1, 2], {recurseIntoArrays: true, arrayMergeMode: 'spread'}> 
// [1, 2, ...string[]]

This is how it behaves in this PR, but is this according to the spec?
I couldn't find any test cases that verify this, even in the original codebase.

I think this might be somewhat related to this test case, though I'm not certain.

// Should merge array into tuple with object entries
type FooNumberTuple = [Foo[], number[]];
type BarArray2D = Bar[][];

declare const fooNumberTupleBarArray2DSpread: MergeDeep<FooNumberTuple, BarArray2D, {arrayMergeMode: 'spread'; recurseIntoArrays: true}>;
expectType<[FooBarSpread[], Array<number | Bar>, ...BarArray2D]>(fooNumberTupleBarArray2DSpread);

declare const fooNumberTupleBarArray2DReplace: MergeDeep<FooNumberTuple, BarArray2D, {arrayMergeMode: 'replace'; recurseIntoArrays: true}>;
expectType<[FooBarReplace[], Bar[], ...BarArray2D]>(fooNumberTupleBarArray2DReplace);

I'll add A to D test cases if they satisfy the spec.

@taiyakihitotsu taiyakihitotsu changed the title MergeDeep producing never[] when merging empty tuples with MergeDeep : fix producing never[] when merging empty tuples. Jan 23, 2026
@taiyakihitotsu taiyakihitotsu changed the title MergeDeep : fix producing never[] when merging empty tuples. MergeDeep : fix producing [never] when merging empty tuples. Jan 23, 2026
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som-sm commented Jan 24, 2026

I use IsEqual because this passes uncorrectly on editor (it rejects correctly with npm test though).

expectType<{f: [0]}>({} as {f: [never]})

@taiyakihitotsu As long as it works with npm test, that's fine. This is a known limitation with using expectType, you won't get errors in the editor for something like:

expectType<string | number>({} as string);

This behavior already exists in several parts of the codebase, so it's fine to use expectType for tests.

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taiyakihitotsu commented Jan 24, 2026

@som-sm

Thanks!

I didn't have a strong conviction regarding the behavior of expectType or how it should be used in type-fest.
That’s why I had been writing the tests via IsEqual (even in my previous PRs)

Now I reverted the tests style.

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MergeDeep with recurseIntoArrays cannot merge an array from source into target when target array is empty

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