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

[mdjermanovic]

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

[mysticatea]

Thank you for your report.

I confirmed it. I prefer that the rule fixes that code along with wrapping by parentheses.

---

In the spec,

• RelationalExpression production requires [In] parameter with positive to allow the in operator.
• In IterationStatement production, the initialization part of for statement passes negative to [In] argument.
• In ArrowFunction production, the bare function body passes the received value to [In] argument as is. On the other hand, the block function body doesn't pass [In] argument. I guess that it means the[In] argument becomes the default value (positive).
• ParenthesizedExpression production passes positive to [In] argument always.

Therefore, for (let a = b => b in c; ;); should be a syntax error.

[mysticatea]

By the way, for (let a = b => [b in c]; ;); is valid.

[mdjermanovic]

I prefer that the rule fixes that code along with wrapping by parentheses.

Where should the fix put parentheses, there are several options.

For example, if the code that should be fixed is:

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

parens could be put around the whole function:

for (let a = ((b, c, d) => b && c in d); ;);

or around the body:

for (let a = (b, c, d) => (b && c in d); ;);

or around the 'in' node':

for (let a = (b, c, d) => b && (c in d); ;);

Depending on the precedence, there could be more expressions inside the parens around the 'in' node.

[mdjermanovic]

I checked how the fix works for cases like () => {return {}}, () => {return {}.a} etc.

The added parentheses are always around the whole body, so that option would be the consistent one:

for (let a = (b, c, d) => (b && c in d); ;);

[eslint-deprecated]

Unfortunately, it looks like there wasn't enough interest from the team
or community to implement this change. While we wish we'd be able to
accommodate everyone's requests, we do need to prioritize. We've found
that accepted issues failing to be implemented after 90 days tend to
never be implemented, and as such, we close those issues.
This doesn't mean the idea isn't interesting or useful, just that it's
not something the team can commit to.

Thanks for contributing to ESLint and we appreciate your understanding.

[mdjermanovic]

I'll fix this.

[eslint-deprecated]

Unfortunately, it looks like there wasn't enough interest from the team
or community to implement this change. While we wish we'd be able to
accommodate everyone's requests, we do need to prioritize. We've found
that accepted issues failing to be implemented after 90 days tend to
never be implemented, and as such, we close those issues.
This doesn't mean the idea isn't interesting or useful, just that it's
not something the team can commit to.

Thanks for contributing to ESLint and we appreciate your understanding.

[anikethsaha]

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

[mdjermanovic]

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