Don't add trailing comma in object rest spread

[duailibe]

For some reason the rest property in objects in typescript is a ExperimentalRestProperty node in the AST and we were not checking for it when printing trailing commas in object expressions.

Fixes #3311

[ikatyang]

I'm still not sure what we should do here since the --trailing-comma all docs said:

"all" - Trailing commas wherever possible (including function arguments). This requires node 8 or a transform.

and the trailing comma is valid in TypeScript.

---

And if the new TS/JS spec allowed more trailing comma cases, how should we deal with it?

EDIT: maybe more es6, es7, es8, ts2.5, ts.2.6, ts2.7 options? 😂

[duailibe]

I'm not opposed to leaving it as is, in that case we should add to arrays and functions.. it's inconsistent right now.

I'm not sure the added complexity is worth though (checking the options + checking the parser) because they're not useful at all in those cases IMO.

EDIT: I understand your point now.. I guess the es5 option should only apply when using babylon? And then we should treat trailingComma as a boolean for TS/flow.

EDIT: maybe more es6, es7, es8, ts2.5, ts.2.6, ts2.7 options? 😂

Don't forget about flow!

[duailibe]

Although leaving this comma would be strictly correct considering "apply wherever possible", I think it wouldn't add much value and we can reason about new cases separately if/when they appear.

[ikatyang]

Since the three trailing comma cases below are all valid TypeScript and we didn't print trailing commas for two of them, removing the trailing comma in the last case should be fine and more consistent.

Prettier 1.8.2
Playground link

--parser typescript
--trailing-comma all

Input:

const [
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest,
] = something;

const {
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest,
} = something;

function func(
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest,
) {
  // something
}

Output:

const [
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest
] = something;

const {
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest,
} = something;

function func(
  longKeySoThisWillGoOnMultipleLines,
  longKeySoThisWillGoOnMultipleLines2,
  longKeySoThisWillGoOnMultipleLines3,
  ...rest
) {
  // something
}