Add ArrayAt type

[WeiWen1999]

closed #369

[sindresorhus]

@giao66 Would you be able to fix the merge conflicts?

[Emiyaaaaa]

Maybe you can refer this PR: #1085

[sindresorhus]

There are still unhandled feedback.

Resolve the comments when you are sure they are resolved.

[WeiWen1999]

There are still unhandled feedback.

Resolve the comments when you are sure they are resolved.

HELLO! I DONE

[sindresorhus]

You force-pushed over my changes.

[WeiWen1999]

You force-pushed over my changes.

So I should sync the main branch first before pushing, right?

[WeiWen1999]

You force-pushed over my changes.

Sorry，This is my first time trying to contribute to an open-source project.

[WeiWen1999]

You force-pushed over my changes.

Can this version be restored?

[sindresorhus]

It's fine. I will do the changes again after merging.

[sindresorhus]

AI review:

---

1. Stray debug/types in the public file

   • source/array-at.d.ts contains scratch types like type TrailingSpreadArray2, type MiddleSpreadArray, type A/B/C, etc. These should never ship in a public .d.ts. Remove them. ([GitHub]1)

2. Dead import(s)

   • GreaterThan is imported but not used (I only see GreaterThanOrEqual/LessThan/LessThanOrEqual being referenced). Drop it. Keep imports minimal to reduce checker work. ([GitHub]1)

3. Unknown index handling (generic N)

   • Top level signature allows N extends number. If N is not a literal, comparisons like IsNegative<N>/LessThan<N, …> become indeterminate and you get brittle distributive behavior or over-narrowing.

   • Add a fast path: for unknown N, return a safe union.

     • For fixed-length tuples: number extends N ? (IsExactOptionalPropertyTypesEnabled extends true ? Required<T>[number] : T[number]) | undefined : …
     • For non-fixed arrays: number extends N ? T[number] : …

   • This mirrors runtime: unknown index => “some element (maybe) or undefined”. It also prevents the type from collapsing to never in some branches.

Potential logic gaps to double-check

• Fixed tuple, out-of-bounds negative: The Sum<T['length'], N> branch correctly yields undefined when the computed index is still negative. Keep that, but cover it with tsd (e.g., ArrayAt<[1, 2, 3], -4> -> undefined). ([GitHub]1)
• Trailing spread, negative indices: The unioning via the “slice then …infer Last” approach looks right (e.g., [A, B, ...C[]], -1 => B | C | undefined depending on spread arity), but you must assert it with tests—especially for -2, -3, etc. where unions widen quickly. ([GitHub]1)
• Leading spread, positive indices: The positive branch unions the possible holders for that position: ArraySlice<StaticPart, 0, N+1>[number] | VariablePart[number] | (N≥len ? undefined : never). This is conceptually correct; again, codify with tests. ([GitHub]1)
• Middle spread recursion: The recursion to NonFixedLengthArrayAt<[...VariablePart, ...TrailingStaticPart], …> (for positive) and the symmetric negative handling looking into TrailingStaticPart is good, but performance-sensitive. Make sure tsd stays fast on large tuples (see perf note below). ([GitHub]1)

Tests you’re missing (add to test-d/array-at.ts)

Cover each of these explicitly with tsd assertions:

Fixed-length tuples

• ArrayAt<[], 0> -> undefined

• ArrayAt<[A], 0> -> A

• ArrayAt<[A], -1> -> A

• ArrayAt<[A, B, C], -1> -> C

• ArrayAt<[A, B, C], -4> -> undefined

• Optional elements with and without exactOptionalPropertyTypes:

  • ArrayAt<[A?, B], 0> under both modes
  • ArrayAt<[A?, B?], -1> under both modes

Non-fixed arrays

• ArrayAt<string[], 0> -> string
• ArrayAt<string[], -1> -> string | undefined (document the choice you want; if you keep it as just string, state why)
• ArrayAt<readonly number[], 1> -> number (readonly preservation)

Spread patterns

• Leading: ArrayAt<[...string[], A, B], 0 | 1 | 2 | -1>
• Middle: ArrayAt<[A, ...string[], B], 0 | 1 | 2 | -1 | -2>
• Trailing: ArrayAt<[A, B, ...string[]], -1 | -2 | -3>
• Mixed optional statics like [A?, ...string[], B?] near boundaries

Unknown index

• ArrayAt<[A, B, C], number> – should be A | B | C | undefined (or at least A | B | C; pick and document)
• ArrayAt<string[], number> – string

Cleanups / polish

• Remove the stray scratch types from source/array-at.d.ts. ([GitHub]1)
• Remove unused GreaterThan import. ([GitHub]1)
• Consider extracting tiny helpers to reduce repetition (NormalizeIndex<T,N>, etc.) if it doesn’t regress checker perf.

Docs

• README entry is fine, but add one more example showing a negative index on a tuple and on a trailing-spread case, plus a note that for non-fixed arrays the result is a union (and may or may not include undefined, per your chosen semantics). ([GitHub]1)

Perf

• This kind of type explodes unions quickly. Run the usual tsc --extendedDiagnostics locally on the repo’s test suite once you add the broader cases. If anything spikes, prefer fewer nested extends infer steps in the hot branches.

---

TL;DR required changes before merge

• Delete the debug types from source/array-at.d.ts. Blocker. ([GitHub]1)
• Drop unused GreaterThan import. Blocker. ([GitHub]1)
• Add an explicit unknown-index guard for N extends number (see above). Strongly recommended.
• Add the missing tsd cases outlined above, especially around exactOptionalPropertyTypes. Strongly recommended. ([TypeScript]2)

[sindresorhus]

After doing the above, I recommend asking ChatGPT to review your pull request and ask it to add more tests if needed.

[WeiWen1999]

After doing the above, I recommend asking ChatGPT to review your pull request and ask it to add more tests if needed.

ok,wizard.Thanks a lot!

[sindresorhus]

• Wrong Every docs (copy-paste). The block under export type Every… documents CollapseRestElement with unrelated examples. Replace with correct description and examples for Every<TArray, Type>. ([GitHub][1])

• Misuse of IsEqual (no type args). In StaticPartOfArray, you wrote IsEqual extends true without parameters, which is invalid. Use an actual comparison, e.g., compare the head optional element:

  type HeadOptional =
  	OptionalPartOfStaticArray<T> extends [infer O] ? O : never;
  
  type HasOptionalHead = IsEqual<HeadOptional, undefined>;
  // …
  : HasOptionalHead extends true ? Result : [...Result, Required<T>[0]?]

(Adjust to your existing generics for T, Result.) ([GitHub][1])

• Duplicate index computation (leading-spread, negative index). In NonFixedLengthArrayAt’s leading-spread negative branch you compute Sum<StaticPart['length'], N> extends infer Index twice. Compute once and reuse. ([GitHub][1])

• Doc wording mismatch. LeadingStaticPartOfMiddleSpreadArray comment says “leading variable, non-fixed-length portion,” but the name is “StaticPart.” Change “variable” → “static” to avoid confusion. ([GitHub][1])

• Missing internal re-exports. source/array-at.d.ts imports many helpers from ./internal/index.d.ts, but this PR doesn’t add those exports to that aggregator. Add them or import directly from ./internal/array.d.ts to prevent broken imports. ([GitHub][1])

• Examples in ArrayAt JSDoc are wrong/incomplete. They show type B = ArrayAt; without type parameters. Use concrete, compilable examples (both positive and negative indices):

  type B = ArrayAt<[string, number, boolean], 0>;  // string
  type C = ArrayAt<[string, number, boolean], -1>; // boolean
  type D = ArrayAt<string[], -1>;                  // string | undefined
  ``` :contentReference[oaicite:5]{index=5}

• Over-defensive conditional in fixed-tuple branch. Index extends unknown ? … : never is redundant (always true). Drop the extra layer for readability.

• Test coverage gaps. Add:

  • readonly tuples.
  • Union index literals (0 | 2, -1 | -3).
  • Union of array kinds (string[] | number[]).
  • Negative out-of-bounds on empty tuple (ArrayAt<[], -1>).
    Current tests cover many shapes but miss these. ([GitHub][1])

• Minor housekeeping. Keep the index.d.ts export grouped with other array utilities (ArraySlice, ArraySplice) for discoverability.