Uncertain De‑precation Timeline for Legacy .eslintrc in ESLint 9

Answer

ESLint has not announced a specific version or date for removing legacy .eslintrc support. Legacy files continue to work in the ESLint 9 line alongside flat config, so treat removal as a future major-version event rather than an ESLint 9.x change. There is no committed deprecation cutoff to wait for.

Confirmed facts versus likely explanation

Confirmed: ESLint 9 defaults to flat config while still reading legacy .eslintrc files. Flat config uses an ordered array of configuration objects instead of a directory-based cascade, and flat config does not automatically read .eslintignore.

Likely explanation, not a commitment: ESLint is preserving legacy support through v9 to permit gradual migration. Because no cutoff is published, schedule migration now instead of waiting for a deprecation notice.

Mapping cascading rule overrides to flat config

The legacy nearest-file-wins directory cascade does not map one-to-one. In flat config, later matching entries generally override earlier ones for the same file and rule. Express each override level as explicit files globs, ordered from broad to specific.

import js from "@eslint/js";
import tseslint from "typescript-eslint";

export default [
  js.configs.recommended,
  ...tseslint.configs.recommended,
  {
    files: ["**/*.ts"],
    rules: { "@typescript-eslint/no-unused-vars": "warn" },
  },
  {
    files: ["src/**/*.ts"],
    rules: { "@typescript-eslint/no-unused-vars": "error" },
  },
  {
    files: ["src/**/legacy.ts"],
    rules: { "@typescript-eslint/no-unused-vars": "off" },
  },
];

Handling ignore patterns

Flat config does not automatically import .eslintignore. Add an explicit ignores array, typically in a broad object first, covering node_modules, build outputs, and generated files. Verify whether your installed ESLint 9 patch excludes node_modules by default before relying on it.

export default [
  {
    ignores: ["node_modules/**", "dist/**", "build/**", "*.config.js"],
  },
  // configuration objects follow
];

Migration steps

1. Run eslint --version and list .eslintrc.*, .eslintignore, and eslint.config.* files to determine which config system is active.
2. Add eslint.config.js or eslint.config.mjs with broad plugin configs first, then file-specific overrides and an ignores object.
3. Run eslint --print-config <file> on representative files before and after migration; compare merged rules, languageOptions, plugins, and effective ignores.
4. Lint a previously ignored path and a file with overridden rules to confirm behavior, then run lint CI.
5. Keep legacy files until flat-config CI passes, then remove .eslintrc.* and .eslintignore.

Missing diagnostic

Provide your exact eslint --version and whether ESLINT_USE_FLAT_CONFIG, legacy .eslintrc.*, .eslintignore, or eslint.config.* files exist. That detail determines whether a gradual per-directory migration or a single flat-config rewrite is appropriate.

Assumptions and uncertainty

• This answer assumes ESLint 9.x, not a pre-release or ESLint 10.
• The legacy removal version is unknown; do not treat any specific future version as confirmed.
• Default node_modules exclusion may vary by patch version; verify per installation.