Get operation on private accessor without getter should throw

[mkubilayk]

Bug Report

• I would like to work on a fix!

Current behavior

• REPL

Transpiled code doesn't throw an error in foo() and returns undefined.

Input Code

class C {
  foo() { return this.#bar; }
  set #bar(a) {}
}

new C().foo();

Expected behavior

new C().foo(); should throw a TypeError.

• Example from Google Chrome Version 90.0.4393.0 Canary:

  VM490:2 Uncaught TypeError: '#bar' was defined without a getter
      at C.foo (<anonymous>:2:11)
      at <anonymous>:6:9
  

• Related test262 test: get-access-of-missing-private-getter.js.

Babel Configuration: See the REPL link

Environment

• Babel version(s): v7.12.12
• Node/npm version: 10.13
• OS: Using REPL
• Monorepo: Using REPL
• How you are using Babel: Using REPL

Possible Solution

_classPrivateFieldGet helper should not fallback to returning descriptor.value if descriptor.get is false-y. If get property exists but is undefined, it can throw this particular error.

Additional context

See PrivateFieldGet-6.b from the spec:

PrivateFieldGet ( P, O )
    1. Assert: P is a Private Name.
    2. If O is not an object, throw a TypeError exception.
    3. If P.[[Kind]] is "field",
        a. Let entry be PrivateFieldFind(P, O).
        b. If entry is empty, throw a TypeError exception.
        c. Return entry.[[PrivateFieldValue]].
    4. Perform ? PrivateBrandCheck(O, P).
    5. If P.[[Kind]] is "method",
        a. Return P.[[Value]].
    6. Else,
        a. Assert: P.[[Kind]] is "accessor".
        b. If P does not have a [[Get]] field, throw a TypeError exception.
        c. Let getter be P.[[Get]].
        d. Return ? Call(getter, O).

[babel-bot]

Hey @mkubilayk! 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."

[jridgewell]

Sigh, another difference with normal properties.

[nicolo-ribaudo]

We can inject the error in a new helper (we already have readOnlyError, and we can create writeOnlyError): it's easy to detect this case at build time, and by doing so we don't unnecessarly increase the helper size for 99.9% of our users.

babel/packages/babel-helper-create-class-features-plugin/src/fields.js

Lines 227 to 232 in 8fcba6e

	if (isAccessor) {
	return t.callExpression(file.addHelper("classPrivateFieldGet"), [
	this.receiver(member),
	t.cloneNode(id),
	]);
	}

Instead of checking if isAccessor, we can:

1. If it's a getter (if (getId)), we inject _classPrivateFieldGet(...)
2. Otherwise, if it's a setter, we inject _writeOnlyError("#bar").

We need to be careful when introducing a new helper, because it will throw an error if we try to inject it with older @babel/core versions. Instead of just doing file.addHelper, we should check if file.availableHelper("writeOnlyError")

I'll mark this as a good first issue: it's not "easy", but it should be self-contained enough.
@mkubilayk If you want to work on this it's yours (two days to claim it!), otherwise it's up for grab by whoever is interested 🙂.

---

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 there 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. Update the test at packages/babel-plugin-proposal-private-methods/test/fixtures/accessors/get-only-setter (delete output.js and it will be automatically generated)
8. Update the code!
9. yarn jest private-methods 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 private-methods 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!

[mkubilayk]

@nicolo-ribaudo writeOnlyError sounds good. I don't have the bandwidth to pick this up right now so happy for someone else to take it. 🙂

[fedeci]

I'll take this, seems to be a quick fix.