Private Name in the left ForOfStatement is not transformed

Bug Report

• I would like to work on a fix!

Current Behavior The following snippet, when transformed via preset-env and stage-3 preset, will throw

Property left of ForOfStatement expected node to be of a type ["VariableDeclaration","LVal"] but instead got "CallExpression"

Input Code

• REPL

class C {
  #p;
  m() {
    for (this.#p of []);
  }
}

Expected behavior/code It should be transformed into something like

// helpers is omissioned
var C = /*#__PURE__*/function () {
  function C() {
    _classCallCheck(this, C);

    _p.set(this, {
      writable: true,
      value: void 0
    });
  }

  _createClass(C, [{
    key: "m",
    value: function m() {
      for (var _i = 0, _arr = []; _i < _arr.length; _i++) {
        i = _arr[_i];
        _classPrivateFieldSet(this, _p, i);
      };
    }
  }]);

  return C;
}();

Possible Solution

As Justin noted, we can fix it by detecting such patterns in @babel/helper-member-expression-to-functions, so babel will apply destructureSet private name handler, which will return a member expression – a valid LHS.

https://github.com/babel/babel/blob/1ba41f208430bdf382b75b56e9aad7436ca522ad/packages/babel-helper-member-expression-to-functions/src/index.js#L110

If you don’t know how to clone 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
7. Add a test (only input.js; output.js will be automatically generated)
8. Update the code!
9. yarn jest babel-plugin-proposal-class-properties 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
10. If it is working, run make test to run all the tests
11. Run git push and open a PR!

Additional context/Screenshots This issue is likely due to plugin ordering: the private-class-properties runs before for-of-transforms after this.#p is transformed to helper calls which is not a valid LHS, the for-of transforms throws on unsupported for-of left.

[jridgewell]

No problem! This isn’t to make you feel guilty, I just want to avoid situation where you’re stuck and haven’t asked for help. Take the time you need, especially with all circumstances going on.

[jridgewell]

Looks like we just need to use the destructureSet transform when inside of the init. This could be a GFI.