Enabling the fix flag for stdio

The problem

I’m using SublimeLinter-contrib-eslint and I noticed that it operates on stdio. This is probably because the linter also works on non-yet-saved-files. I wanted to enable autofixing of the code, but it was not built-in and when manually passing the --fix flag I saw that ESLint just exits with an error message. I decided that enabling --fix and --stdin together was the right first step to autofixing with the linter plugin.

The solution

Currently there are three proposed solutions:

1. The one I’ve prepared a PR for (#9073), that utilizes a non-standard file descriptor (number 3, just after 0, 1, 2 for stdin, stdout, stderr respectively) for providing the fixed output.
2. The one that comes from @not-an-aardvark, that suggests just removing the existing restriction and allowing the use of a custom formatter to retrieve the output.
3. Add a new flag like --fix-dry-run that can be used with --stdin, but doesn’t output fixes to the filesystem.

I’d be happy to go with either of them, and frankly, the second one feels like a more convenient option to use.

(edited by @not-an-aardvark to add the third proposed solution)

[not-an-aardvark]

@platinumazure Any thoughts on the updated proposal in https://github.com/eslint/eslint/issues/9076#issuecomment-320484888? If that proposal addresses your objections, then I’d like to add this to the TSC agenda for approval. Otherwise, we can discuss more to hopefully find the best solution.

[not-an-aardvark]

I’m addressing this - not everything is fixable, so if i wanted to get both the warnings/errors and the fixed output at the same time, I’d need to run ESLint twice. Or not?

No – to clarify, by “output” I mean the object that is usually passed to a formatter, which looks something like this:

Result list

[
  {
    "filePath": "/path/to/foo.js",
    "messages": [
      {
        "ruleId": "no-undef",
        "severity": 2,
        "message": "'foo' is not defined.",
        "line": 1,
        "column": 1,
        "nodeType": "Identifier",
        "source": "foo;",
        "endLine": 1,
        "endColumn": 4
      }
    ],
    "errorCount": 1,
    "warningCount": 0,
    "fixableErrorCount": 0,
    "fixableWarningCount": 0,
    "output": "foo;\n"
  }
]

Note that this includes the autofix output in the "output" property. (This output came from a run where the no-undef and semi rules were enabled, and the semi problem was autofixed, so only the no-undef problem was included in the problem list.)

Could I use --stdin and --fix-dry-run together with a custom formatter to get everything in one go?

Yes, that’s the idea. You wouldn’t even need a custom formatter – you could use the built-in json formatter, which just takes objects like the one above and outputs them as JSON.