type-challenges-solutions/en/easy-tuple-length about type-challenges-solutions HOT 7 OPEN

This doesn't work with a new expected error test case, because strings also have a .length property.

  // @ts-expect-error
  Length<'hello world'>,

This will pass the current specs by requiring an array:

type Length<T extends readonly any[]> = T['length']

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@hpr While the solution is correct, I've to slightly extend the reasoning.
If it was just excluding strings, the following would also work:

type Length<T extends any[]> = T['length']

But it fails because it expects readonly types. So the solution provided by @hpr works.

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My take was this one to avoid the strings

type Length<T> = T extends readonly any[] ? T['length'] : never 

After that I notice that the constraint can be set in the generic too

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Based on the positive test cases:

Expect<Equal<Length<typeof tesla>, 4>>,
Expect<Equal<Length<typeof spaceX>, 5>>

Note the typeof tesla instead of tesla, the current solution no longer works.

And regarding the negative test cases:

// @ts-expect-error Length<5>,
// @ts-expect-error Length<'hello world'>

If I understand correctly, since 5 and 'hello world' needs to throw an error, we then need to limit T within generic, rather than in the assignment expression (to the right of =). Hence the solution should be:

type Length<T extends readonly unknown[]> = T['length'];

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@hpr you are right. Having a constraint over an array seems like more natural even, comparing to my original solution.

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@ghaiklor Can you update your solution with ⬇️

type Length<T extends readonly any[]> = T['length'];

This will be usefull for other users.

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@AurelienWebnation feel free to add a PR with the proposed update. As I see it, it can be placed after the original solution with the note that there is an alternative solution.

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