ReadonlyArray with length should be narrowed in noUncheckedIndexedAccess #62719

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1. MartinJohns commented on Nov 5, 2025

   

   Martin Johns (MartinJohns)

   on Nov 5, 2025

   Contributor

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   Duplicate of #53678.

   I want to define a fixed length array and access if safely using noUncheckedIndexedAccess

   A fixed-length array would be a tuple, and if you use a tuple it works.

   ---

   I highly doubt your suggestion would get improved, because a viable workaround exists (use tuples), the intersection with the explicit length property has no effect on narrowing the ReadonlyArray<number> and the team expressed multiple times that they don't have any intention on improving the behavior of noUncheckedIndexedAccess.
2. hyunbinseo commented on Nov 5, 2025

   

   Hyunbin Seo (hyunbinseo)

   on Nov 5, 2025

   Author

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   If so, I guess a Tuple utility would be the only workaround:

   type Tuple<T, N extends number> = N extends N
   	? number extends N
   		? T[]
   		: _TupleOf<T, N, []>
   	: never;
   
   type _TupleOf<T, N extends number, R extends unknown[]> = R['length'] extends N
   	? R
   	: _TupleOf<T, N, [T, ...R]>;
   
   // [string, string, string, string, string]
   type String5 = Tuple<string, 5>;
3. Hyunbin Seo (hyunbinseo)

   closed this as completedon Nov 5, 2025
4. microsoft

   locked as resolved and limited conversation to collaborators on Jan 7, 2026