eslint 9.x

Complete instructions

ESLint 9 executable lint workflows

Read the installed ESLint, Node and plugin versions. The fixture targets ESLint 9.39.5 with typescript-eslint 8.70.0 and TypeScript 5.9.3. Preserve an existing configuration's scope; migrating legacy config is a separate task.

Upstream version support lists ESLint 9 as end of life since 2026-08-06. This guide supports an existing pinned 9.x application during maintenance or migration; it does not recommend installing that unsupported major in a new project. Prefer a currently supported release when Node and plugin compatibility allow, and plan existing-pin upgrades separately from ordinary lint fixes.

Use flat config deliberately

ESLint 9 uses flat config by default. Define language options, plugins and rule configuration as objects rather than translating legacy extends strings mechanically. Recent ESLint 9 versions supply defineConfig and its scoped extends:

import { defineConfig } from "eslint/config";

export default defineConfig({
  files: ["src/**/*.{js,mjs}"],
  rules: { "no-unused-vars": "error" },
});

Verify installed-patch support when working on an older 9.x release. An object containing only ignores represents a global ignore; an ignore on an object with other configuration keys scopes that object's application. Check the actual file's effective config before broadening a glob.

eslint --print-config src/example.js
eslint src --max-warnings=0

Missing rules can result from file selection, plugin registration or config lookup. A parser accepting TypeScript does not enable type-aware rules; those need project information from typescript-eslint.

Typed linting

Use a scoped TypeScript config through defineConfig with the installed preset:

import { defineConfig } from "eslint/config";
import tseslint from "typescript-eslint";

export default defineConfig({
  files: ["src/**/*.ts"],
  extends: [tseslint.configs.recommendedTypeChecked],
  languageOptions: {
    parserOptions: {
      projectService: true,
      tsconfigRootDir: import.meta.dirname,
    },
  },
});

Keep these files in the intended tsconfig. Out-of-project files need a deliberate separate untyped config or a narrowly allowed default project; do not load every workspace file into one fallback project. Use the application's TypeScript compiler check as well; linting is not a substitute.

Verify diagnostics and fixes

Run the installed CLI on intended paths. Exit 1 means lint violations; exit 2 means a configuration or execution failure. Report rule identifiers and locations rather than treating every nonzero exit as the same failure.

Preview an unfamiliar fix with --fix-dry-run and a suitable formatter, review its scope, then use --fix when the task authorizes the edit. Re-run lint on the written files. A fix does not necessarily resolve type errors or application behavior.

The fixture checks flat file selection, a typed floating-promise violation, a bounded fix and clean revalidation while malformed ignored input is skipped. The formatting rule in that fixture is an existing-rule compatibility check, not a recommendation to replace the project's formatter.

Sources

• ESLint 9 migration
• Configuration files
• Command-line interface
• Typed linting