Typescript: Trailing comma on final parameter missing when ... used

[alantreadway]

Prettier 1.12.1
Playground link

--parser typescript
--trailing-comma all

Input:

export function testFixture(
  {
    eventName = <'MODIFY'|'DELETE'> 'MODIFY',
    eventSourceARN = faker.internet.url(),
  } = {},
  ...records: {}[]
): Event {
  // Do some processing
}

Output:

export function testFixture(
  {
    eventName = <"MODIFY" | "DELETE">"MODIFY",
    eventSourceARN = faker.internet.url(),
  } = {},
  ...records: {}[]
): Event {
  // Do some processing
}

Expected behaviour:

export function testFixture(
  {
    eventName = <"MODIFY" | "DELETE">"MODIFY",
    eventSourceARN = faker.internet.url(),
  } = {},
  ...records: {}[],
): Event {
  // Do some processing
}

[duailibe]

This is intended behavior, to align with the spec even though TypeScript allows it. Shouldn't make a difference since you can't have another argument after it anyway, you won't have any "diff issues" (which is the whole point of having trailing commas).

Duplicate of #3673

[bakkot]

(@duailibe, technically this is about function trailing commas, not object-rest-spread, but it's the same deal.)

Incidentally, TypeScript will disallow this syntax next time a release is cut from master.

[duailibe]

@bakkot Thank you, I had another issue opened, last minute I realized it was about objects and forgot to fix the link to the spec. I removed it anyways

[alantreadway]

Thanks for the clarifications, SGTM if they are about to disable this in the compiler for TS 2.9