ESLint v9 Flat Config: Unresolved Rule Merging with Shareable Configs

Goal

Determine how rules are applied when a rule appears in both a root eslint.config.js and an imported shareable config under ESLint v9’s flat configuration system.

Constraints & Uncertainty

• ESLint v9 defaults to flat config and no longer loads legacy .eslintrc.* files automatically.
• Flat config requires explicit imports for plugins, parsers, and shareable configs.
• Rule merging semantics differ from the cascading overrides of the legacy .eslintrc format.
• The documentation does not clearly state whether the root config overrides the shareable config or vice versa when the same rule is defined in both.

Specific Questions

1. When a rule is defined in both the root flat config and an imported shareable config, which definition takes precedence?
2. If both definitions use different rule options, how are those options combined or overridden?
3. Does the order of imports in the flat config array affect rule precedence relative to the root config?