Type widening issue with `.filter(Boolean)`

Question

Type widening issue with `.filter(Boolean)`

Closed this issue a year ago · 5 comments

Is this expected?

type A = {
  a: string
}

const as1: A[] = [
  { a: "1" },
  { a: "2", b: false },
         // ^ Type '{ a: string; b: false; }' is not assignable to type 'A'.
         //     Object literal may only specify known properties, and 'b' does not exist in type 'A'.
]

const as2: A[] = [
  { a: "1" },
  { a: "2", b: false }, // no error
].filter(Boolean)

TS Playground

Answer 1 · 2023-08-24T12:52:08.000Z

Doesn't look like ts-reset is to blame here!

Answer 2 · 2023-09-02T16:51:54.000Z

@mattpocock could you elaborate please?

Answer 3 · 2023-09-03T08:01:22.000Z

Try it without ts-reset installed - same result.

Answer 4 · 2023-09-03T14:35:54.000Z

But without ts-reset, the base .filter(Boolean) improvements also have same result.... I'm asking why this could not also be improved? We can both see it could do a better job here, right? Is this just not possible?

…

On Sun, Sep 3, 2023 at 1:01 AM Matt Pocock ***@***.***> wrote: Try it without ts-reset installed - same result. — Reply to this email directly, view it on GitHub <#163 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAA2VKAOWO27OE5O6LQYEA3XYQ2NZANCNFSM6AAAAAA3ZI44V4> . You are receiving this because you authored the thread.Message ID: ***@***.***>

Answer 5 · 2023-09-04T10:42:16.000Z

Right - no, it isn't, because of the way that TS treats excess property checking.