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Bad error message for assigning empty array to tuple type #6524
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Nathan Shively-Sanders (@sandersn) Daniel Rosenwasser (@DanielRosenwasser) would this be fixed by the proposed contextual typing changes?
If I understand things correctly, you should get
number[]with Nathan's changes.Even if you didn't, I think I've concluded on several occasions with Jason Freeman (@JsonFreeman) and Cyrus Najmabadi (@CyrusNajmabadi) that surfacing the
undefinedtype in error messages is not a problem.Even if you didn't, I think I've concluded on several occasions with Jason Freeman (@JsonFreeman) and Cyrus Najmabadi (@CyrusNajmabadi) that surfacing the
undefinedtype in error messages is not a problem.You don't think it gives the impression of being a compiler bug?
That's what I argued at the time because it totally looks like we screwed up somehow. The fact that we're surfacing the
undefinedtype isn't a problem so much as it is that its representation kinda, well, sucks. 😄I checked, looks like the error with the change in #6118 is still the same.
Is there such thing as a tuple of length 0? It seems like the error ought to say something like
type [] is not assignable to type [number]
where
[]is the tuple of length 0. But I guess a tuple of length 0 is really the same asundefined[]anyway, so maybe there is no point in having that concept.This is not to say that the type
undefinedshould never be surfaced in an error message. That is a separate discussion. I think the better question is not aboutundefined, but about how to improve error messages about tuple types. I think that while technically, a tuple type acts as an object with numeric properties, perhaps the error should talk about the "size / length" of the tuple. I think that would be more intuitive for users.Is this still a bug given that the
nullandundefinedtypes are going to be representable in 2.0?(1.9.0-dev.20160402)
foo.ts:
var x = [];
tsc -d -t es5 foo.tsfoo.d.ts:
declare var x: any[];
foo.ts:
var x: [number] = [];
tsc -t es5 foo.tsfoo.ts(1,5): error TS2322: Type 'undefined[]' is not assignable to type '[number]'. Property '0' is missing in type 'undefined[]'.There's no difference with
--strictNullChecks
So is an empty array
any[]orundefined[]? Should it beundefined[]even if--strictNullChecksisn't passed in?For the tuple case, it doesn't really matter what the element type is. The array literal is required to have an element at index 0 because of the contextual type, but it does not.
For the other case, if the array did not get widened to
any[], I don't think you would be able to subsequently add elements to it, which would be very limiting.Both of these ideas are independent of strict null checking. That said, I still think it would be reasonable to have tuples of length 0, and then the message would talk about the type
[]as the empty tuple type.- added this to the milestone
on Apr 26, 2016 We have empty tuples now, and the new message is
Property '0' is missing in type '[]' but required in type '[number]'.- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
Reported in the IRC channel against 1.7.5. Still exists in 1.8.0-dev.20160117
I guess
undefinedcomes from the fact that the RHS array is empty, so its element type is left uninitialized. Maybe you want to detect that and have a different error message? "An empty array is not assignable to type [number]." or something.