arrow-body-style autofix with 'in' in a for-loop init

Tell us about your environment

• ESLint Version: 5.16.0 (same behaviour with 6.0.0-rc.0)
• Node Version: 10.16.0
• npm Version: 6.9.0

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 6,
  },
  rules: {
    "arrow-body-style": "error",
  }
};

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

for (let a = b => { return b in c; }; ;);

eslint index.js --fix

What did you expect to happen?

Not a SyntaxError in the fixed code.

What actually happened? Please include the actual, raw output from ESLint.

for (let a = b => b in c; ;);

The fix changed semantics and produced an error:

SyntaxError: for-in loop variable declaration may not have an initializer.

This is similar to #11706

Are you willing to submit a pull request to fix this bug?

Yes, when PR #11848 is finished, since it might be a similar fix.

Note

The fixed code is SyntaxError in Node/Chrome and Firefox (with a bit different message).

However, it’s a valid ForStatement for acorn, babel-eslint and other parsers (e.g. not esprima).

Check: AST Explorer

Reported acorn #838

[mdjermanovic]

@anikethsaha I’m not working on it at the moment, feel free to take this!

[anikethsaha]

@mdjermanovic are you working on this? if not I would like to take this