feat: Support basePath property in config objects

[mdjermanovic]

Summary

This RFC proposes supporting basePath property in config objects.

When present, config object's basePath overrides config array's basePath, meaning that files and ignores patterns in that config object will be treated as relative to the config object's basePath instead of the config array's basePath.

Related Issues

eslint/eslint#18948

[nzakas]

Presumably this is because we need to combine the basePath with the ignore patterns to get the correct files/directories to ignore? If so, can you add that detail here?

[nzakas]

The reason I thought it might be good to include this feature in this RFC is because of how --base-path might interact with a config basePath property. Would it just replace the config file base path completely such that any relative basePath property would now be relative to the value passed as --base-path?

[fasttime]

I'm thinking about the situation where a parent config and an extended config have different base paths. Per the current defineConfig logic, the files and ignores properties would be merged, but the merged config will inherit the extended config's base path, which seems unexpected.

Maybe relative base paths in extended configs should be joined to the base path of the parent config?

[nzakas]

I'm thinking about the situation where a parent config and an extended config have different base paths.

I'm not sure I understand what you're describing. Can you provide an example?

[fasttime]

Here's an example where the basePath property is manually defined on the parent object:

export default defineConfig({
    basePath: "packages/foo",
    files: ["**/*.js"],
    rules: {
        rule1: "error"
    },
    extends: [{
        ignores: ["ignored.js"],
        rules: {
            rule2: "error"
        }
    }]
});

Using the fork in mdjermanovic/rewrite#1, this will result in the following array:

[
    {
        ignores: ["ignored.js"],
        rules: { rule2: "error" },
        files: ["**/*.js"],
        name: "UserConfig[0] > ExtendedConfig[0]"
    },
    {
        basePath: "packages/foo",
        files: ["**/*.js"],
        rules: { rule1: "error" }
    }
]

Basically the merged config object (the first array element) now contains the files of the parent config without the basePath that should be used to resolve those patterns.