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"Target requires x element(s) but source may have fewer." despite filter by length #62422
Closed as not planned
Description
🔎 Search Terms
Target requires element(s) but source may have fewer, filter, length
🕗 Version & Regression Information
Not a regression
⏯ Playground Link
💻 Code
const arr: string[] = [ "abc", "def", "ge" ];
const trigrams: [string, string, string][] = arr.map(x => x.split("")).filter(x => x.length == 3);
console.log(trigrams);🙁 Actual behavior
Type 'string[][]' is not assignable to type '[string, string, string][]'.
Type 'string[]' is not assignable to type '[string, string, string]'.
Target requires 3 element(s) but source may have fewer.
🙂 Expected behavior
No type error
Additional information about the issue
No response
Activity
jcalz commentedon Sep 9, 2025
note: not a TS team member, ignore me if you want
TS does not treat checking
lengthas a type guard on array types that aren't already (unions of) tuples. This isn't a bug; everything is behaving as intended and as designed. It's more of a limitation or missing feature. Maybe it's a duplicate of #28337.As a workaround you can always tell TS that
x => x.length == 3behaves as a type guard, like so:Playground link
Sainan commentedon Sep 9, 2025
Well, it can be done shorter :^)
Playground link
But the point is that this is unexpected. It's not like a major issue, but definitely something that "could be improved."
RyanCavanaugh commentedon Sep 9, 2025
We don't really have a good way to implement this. Even intersecting in a
{ length: 3 }property (which even by itself is suspect in terms of downstream effects), that's not enough to turn astring[]into a[string, string, string]jcalz commentedon Sep 9, 2025
The point wasn't to code-golf, it was to show how you could tell TS that the filter callback was meant to act as a type guard, without changing the emitted JS at all.
Sainan commentedon Sep 9, 2025
I know. My point was just to show that
.mapcan help it see the type conversion since indexing astring[]is seen as producingstringby default.typescript-bot commentedon Sep 12, 2025
This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
Sainan commentedon Sep 12, 2025
Can
.filter(x => x.length == 3)really not be implied to be the same as.map(x => [ x[0], x[1], x[2] ])?RyanCavanaugh commentedon Sep 12, 2025
We're not really willing to add special-case magic like that to the type system. If something can't be implemented in a way that's general-purpose, it's generally not done at all