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Config File Improvements: extends in overrides

Summary

This proposal makes the configuration in overrides property supporting extends property and overrides property (fixes eslint/eslint#8813).

Motivation

Currently, cascading configuration supports extends, but globa-based configuration doesn't. This lacking has been preventing people to migrate their configuration to glob-based.

To support extends property (and nested overrides property for completeness) in overrides solve the pain.

Detailed Design

Proof of Concept (implementation): https://github.com/eslint/eslint/tree/proof-of-concept/config-array-in-eslintrc

This proposal enhances there.

If a configuration in overrides property has extends property or overrides property, it flattens those recursively rather than throws errors. The files property and excludedFiles property of the configuration is applied to every flattened item. If a flattened item has own files property and excludedFiles property, it composes those by logical AND.

lib/lookup/config-array-factory.js#L601-L624 in PoC.

💡 Example: 
{
    "extends": ["eslint:recommended"],
    "rules": { ... },
    "overrides": [
        {
            "files": ["*.ts"],
            "extends": ["plugin:@typescript-eslint/recommended"],
            "rules": { ... },
        }
    ]
}
is flattend to: 
[
    // extends
    {
        "name": ".eslintrc.json » eslint:recommended",
        "filePath": "node_modules/eslint/conf/eslint-recommended.js",
        "rules": { ... }
    },
    // main
    {
        "name": ".eslintrc.json",
        "filePath": ".eslintrc.json",
        "rules": { ... }
    },
    // overrides (because it flattens recursively, extends in overrides is here)
    {
        "name": ".eslintrc.json#overrides[0] » plugin:@typescript-eslint/recommended",
        "filePath": "node_modules/@typescript-eslint/eslint-plugin/dist/index.js",
        // `matchFile` is merged from the parent `overrides` entry and itself.
        "matchFile": { "includes": ["*.ts"], "excludes": null },
        "parser": { ... },
        "parserOptions": { ... },
        "plugins": { ... },
        "rules": { ... }
    },
    {
        "name": ".eslintrc.json#overrides[0]",
        "filePath": ".eslintrc.json",
        "matchFile": { "includes": ["*.ts"], "excludes": null },
        "rules": { ... }
    }
]

Documentation

This enhancement should be documented in "Configuring ESLint" page.

Drawbacks

  • I think no problem.

Backwards Compatibility Analysis

If people depend on the behavior that ESLint throws an error if they give extends property or overrides property in a config of overrides property, this enhancement breaks that. However, I believe that we don't need to worry.

Alternatives

  • We can provide a utility that merges multiple configs for this purpose. However, people have gotten used to extends property, and it's easy to use.
  • Array Config may be an alternative, but it cannot apply files and excludedFiles to the extended config by logical AND.

Open Questions

Frequently Asked Questions

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