Trailing comma in typescript spread

[jandolezal71]

Bug Report

Hi,

we have problem with trailing comma in typescript after upgrade to @babel/core to 7.7.4.

We installed new prettier (https://github.com/prettier/prettier/blob/master/CHANGELOG.md#1182), which was injected trailing comma after rest in type, but babel-parser don't like it.

tsc command will not produce any errors, so trailing commas are fine in TS.

Cannot be this relevant to "final fix" #10491 ?

Error:

SyntaxError: /my-project/src/file.ts: Unexpected trailing comma after rest element (12:12)

  10 | 	string,
  11 | 	string,
> 12 | 	...string[],
     | 	           ^
  13 | ];
  14 | 
  15 | export default function createChainableTypeChecker<T>(
    at Object.raise (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:6975:17)
    at Object.raiseTrailingCommaAfterRest (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8881:10)
    at Object.checkCommaAfterRest (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8869:14)
    at Object.tsParseTupleElementType (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:4876:12)
    at Object.tsParseDelimitedListWorker (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:4519:23)
    at Object.tsParseDelimitedList (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:4508:25)
    at Object.tsParseBracketedList (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:4554:25)
    at Object.tsParseTupleType (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:4859:30)
    at Object.tsParseNonArrayType (/my-project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:5001:21)

Ts file:

type ValidateArgs = [
	{
		[key: string]: any;
	},
	string,
	string,
	...string[],
];

package.json

    "@babel/core": "^7.7.4",
    "@babel/plugin-proposal-object-rest-spread": "^7.7.4",
    "@babel/preset-env": "^7.7.4",
    "@babel/preset-typescript": "^7.7.4",

• Babel version(s): 7.7.4
• Node/npm version: 12.10
• OS: Ubuntu 19
• Prettier: 1.19.1

[babel-bot]

Hey @jandolezal71! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

[JLHwung]

This issue can reproduced in babel 7.0.0. So it's not a regression.

[nicolo-ribaudo]

I believe that this is a bug in TypeScript itself, and I have opened an issue asking which is the correct behavior.

microsoft/TypeScript#35375

[jandolezal71]

Thank you Nicolò for processing this request.

[nicolo-ribaudo]

If anyone wants to submit a PR, probably deleting the check for the rest comma from the tsParseTupleElementType parser function would be enough, but we'll also need tests.

babel/packages/babel-parser/src/plugins/typescript/index.js

Line 615 in cfd26fd

tsParseTupleElementType(): N.TsType {

If it is the first time that you contribute to Babel, follow these steps: (you need to have make and yarn available on your machine)

1. Write a comment here to let other possible contributors know that you are working on this bug.
2. Fork the repo
3. Run git clone https://github.com/<YOUR_USERNAME>/babel.git && cd babel
4. Run yarn && make bootstrap
5. Wait ⏳
6. Run make watch (or make build whenever you change a file)
7. Add a test (only input.js; output.json will be automatically generated)
8. Update the code!
9. yarn jest babel-parser to run the tests
   • If some test outputs don't match but the new results are correct, you can delete the bad output.json files and run the tests again
   • If you prefer, you can run OVERWRITE=true yarn jest babel-parser and they will be automatically updated.
10. If it is working, run make test to run all the tests
11. Run git push and open a PR!

[yeonjuan]

@nicolo-ribaudo I will pick this up :)

[nicolo-ribaudo]

Sure!