Babel-eslint: Discussion: how do we handle missing Babel config files in v11

[kaicataldo]

From https://github.com/babel/babel-eslint/pull/711#issuecomment-453157175

Original question:

It looks like babelCore#parseSync will still parse without a config file, it just won't enable any plugins. Does this seem like a reasonable default? I'm worried about it being confusing behavior since Babel itself won't run any transformations without a config file. An alternative would be to run a check to see if a config file can be found (not sure if @babel-core provides an API for this already or not) and throw an error with a helpful message, if not.

Sounds like there might issues with any approach that doesn't just parse using defaults, as described here by @loganfsmyth :

The issue is that not all files in your project may have a config. If you have a .babelrc in src/ but didn't bother with any Babel config for the rest of your project, you should still be able to lint those files using the parser's defaults.

I guess we have to decide if babel-eslint is quite literally only for code that is transformed by Babel or if it can also be used for non-transformed code in a project that mostly consists of files that _are_ transformed by Babel. Concrete example I can think of off the top of my head is a tooling config file (.eslintrc.js, babel.config.js, etc.) that might not have a config.

From a philosophical standpoint, I actually don't think it's unreasonable to say that those files should be parsed with the default parser (which will always be available since it's a dependency of ESLint and is totally possible with ESLint's glob-based configuration). Practically speaking, though, I'm not sure it's a good idea. It increases the overhead for _every_ new project that uses babel-eslint and has any non-transformed JS code.

[loganfsmyth]

FWIW despite my previous comment, I'm not against this as a default, as long as there is an opt-out.

We could do:

const cfg = babel.loadPartialConfig(/* ... */);
if (!opts.allowNoConfig && cfg && !cfg.hasFilesystemConfig()) {
  throw new Error(`No Babel config file detected for ${cfg.options.filename}. Either disable config file checking with allowNoConfig:false, or configure Babel so that it can find the config files.`)
}
const ast = cfg && babel.parse(cfg.options);

[kaicataldo]

Practically speaking, though, I'm not sure it's a good idea. It increases the overhead for every new project that uses babel-eslint and has any non-transformed JS code.

I'm starting to wonder if making exceptions for this case would be a mistake. Saying that babel-eslint will only run on code that Babel is set up to transform makes things a lot easier from a maintenance standpoint. typescript-eslint-parser has always worked this way and it hasn't really been an issue.

[kaicataldo]

This was discussed in today's Babel meeting. Notes can be found here.

The solution we landed on is as follows:

Each time babel-eslint parses a file and no corresponding config file is found, an error will be thrown with an optional opt-out. (this should be an edge case)

[danpoq]

I got the error @loganfsmyth proposed above in 11.0.0-beta.0 and had to solve it by passing the location of babel.config.js in parserOptions.babelOptions.configFile because I'm working in a monorepo. This is specifically documented on the README (👍) but just posting this here in case anybody else comes to this issue in the same situation.