[Bug]: error parsing typescript assertion signature

[JonathanMEdwards]

💻

• Would you like to work on a fix?

How are you using Babel?

Other (Next.js, Gatsby, vue-cli, ...)

Input code

this doesn't parse

but this does:

function assert(condition: any): asserts condition {}

Configuration file name

No response

Configuration

No response

Current and expected behavior

parsing should match typescript

Environment

babel repl

Possible solution

No response

Additional context

No response

[babel-bot]

Hey @JonathanMEdwards! 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."

[fedeci]

This bug comes from the tsParseTypePredicateAsserts function where we check that the asserts keyword does not have any line break before it. We should change the behaviour of that function to accept line breaks.

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

Lines 1318 to 1341 in 710b391

	tsParseTypePredicateAsserts(): boolean {
	if (
	!this.match(tt.name) ||
	this.state.value !== "asserts" ||
	this.hasPrecedingLineBreak()
	) {
	return false;
	}
	const containsEsc = this.state.containsEsc;
	this.next();
	if (!this.match(tt.name) && !this.match(tt._this)) {
	return false;
	}
	if (containsEsc) {
	this.raise(
	this.state.lastTokStart,
	Errors.InvalidEscapedReservedWord,
	"asserts",
	);
	}
	return true;
	}

---

If anyone wants to help fixing this bug and 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.ts; output.json will be automatically generated) in https://github.com/babel/babel/tree/main/packages/babel-parser/test/fixtures/typescript/assert-predicate
   • We need a test for the valid case (the example in the bug report) and for the invalid case [function].
8. Update the code!
9. yarn jest parser to run the tests
   • If some test outputs don't match but the new results are correct, you can delete the bad output.js files and run the tests again
   • If you prefer, you can run OVERWRITE=true yarn jest parser and they will be automatically updated.
   • New parser tests sometimes fail: don't worry, run yarn jest parser again after that
10. If it is working, run make test to run all the tests
11. Run git push and open a PR!

[JuniorTour]

Can i take this?

[fedeci]

Sure!