ESLint v9 Flat Config: Unresolved Rule Merging with Shareable Configs

Rule Precedence in Flat Config

In ESLint v9's flat configuration system, rule precedence is determined by the sequential order of the configuration array. The last object in the array that matches a specific file takes precedence over all previous objects.

Precedence and Import Order

Because a shareable config is simply an object (or an array of objects) imported into your eslint.config.js, its precedence depends entirely on where it is placed within the exported array:

• Shareable config first: If you import a shareable config and place it at the beginning of the array, any subsequent objects in the root config that define the same rule will override it.
• Shareable config last: If the shareable config is placed at the end of the array, its rules will override any previous definitions in your root config.

Rule Option Merging

Rule definitions are not deeply merged. When a rule is redefined in a later configuration object, the new definition completely replaces the previous one. This includes both the severity level (e.g., "error") and the options array.

For example, if a shareable config defines a rule as "rule-name": ["error", { "optionA": true }] and your root config defines it as "rule-name": ["warn", { "optionB": true }], the final resolved configuration will be "warn" with only optionB enabled. optionA is discarded entirely.

Verification Method

To verify exactly which rule and options are being applied to a specific file, use the ESLint CLI to print the resolved configuration:

npx eslint --print-config path/to/your/file.js

Diagnostic Detail

To provide a more specific recommendation, please clarify: Are you importing the shareable config as a single object or spreading an array of configs into your root array? (The latter can introduce multiple hidden override layers depending on the shareable config's internal structure.)