ESLint 28 🛡️ Cheatsheet - KandZ

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ESLint v10 is flat-config only. The .eslintrc system is gone, the config resolution starts from each file’s directory, and the Node.js floor is ^20.19.0 || ^22.13.0 || >=24 . The cheatsheet is the set of decisions that keep a v10 config loadable and a rule set predictable.

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The Node.js Gate

Version	Status
`^20.19.0`	Supported
`^22.13.0`	Supported
`>=24`	Supported
`v21`, `v23`	Dropped
`< 20.19.0`	Dropped

Check engines.node and the CI image before touching the config. The install fails at the engine check, not at lint time.

The Flat Config Structure

Legacy (dead)	Flat (v10)
`.eslintrc.*`	`eslint.config.js` / `.mjs` / `.ts`
`extends: ["..."]`	Import and spread, or `defineConfig`
`env: { browser: true }`	`languageOptions.globals`
`.eslintignore`	`globalIgnores()` or `ignores`
`plugins: ["react"]`	`plugins: { react: reactPlugin }`

The defineConfig() helper from eslint/config auto-flattens arrays and provides type safety . The globalIgnores() helper marks a block as global ignores, which is clearer than a bare ignores key .

// eslint.config.mjs — the essential v10 config
import js from "@eslint/js";
import { defineConfig, globalIgnores } from "eslint/config";
import tseslint from "typescript-eslint";
import eslintConfigPrettier from "eslint-config-prettier";

export default defineConfig(
  globalIgnores(["dist/**", "coverage/**"]),
  js.configs.recommended,
  tseslint.configs.recommended,
  eslintConfigPrettier, // last: disables formatting rules
);

The Config Lookup Change

v9 resolved the config from the cwd. v10 resolves it from the directory of each linted file, walking upward . A monorepo with per-package configs works without flags. A project that relied on the cwd lookup can pass --config explicitly.

The v10_config_lookup_from_file flag is removed because the behavior is the default .

The `eslint:recommended` Update

The v10 recommended config includes new rules that were not in v9 . A project that extends js.configs.recommended sees new reports after the upgrade. Address them or disable them explicitly; the reports are the config doing what it was updated to do.

The Rule Changes

Rule	v10 Change
`radix`	`"always"` and `"as-needed"` deprecated; radix always required
`no-shadow-restricted-names`	`globalThis` reported by default
`func-names`	Extra array items invalid
`no-invalid-regexp`	Duplicate `allowConstructorFlags` invalid
`no-unexpected-multiline`	Checks regex literals with `d` and `v` flags

The no-shadow-restricted-names change is the one that produces reports without any config change:

// v10 reports this by default
const globalThis = "foo";

The fix is to rename the variable or set reportGlobalThis: false .

The `eslint-env` Removal

The /* eslint-env */ comments are reported as errors in v10 . Replace them with languageOptions.globals:

// Before (v9)
/* eslint-env serviceworker */

// After (v10)
languageOptions: { globals: { ...globals.serviceworker } }

The Ecosystem Blocker

eslint-plugin-import 2.32.0 does not support ESLint 10 and crashes at plugin-load time with a TypeError about sourceType . The replacement is eslint-plugin-import-x, a drop-in fork with a peer range that includes ^10.0.0 .

Package	Status
`eslint-plugin-import`	Crashes on v10
`eslint-plugin-import-x`	v10-compatible

The swap requires a rule-prefix change from import/ to import-x/, including in eslint-disable comments .

The typescript-eslint Setup

typescript-eslint v8 supports ESLint 10 . The type-aware mode uses projectService: true instead of the older project path .

{
  files: ["**/*.ts", "**/*.tsx"],
  languageOptions: {
    parserOptions: {
      projectService: true,
      tsconfigRootDir: import.meta.dirname,
    },
  },
}

The allowDefaultProject option lints config files that are not in a tsconfig.json .

The Prettier Integration

eslint-config-prettier must be last in the config array. It disables the ESLint formatting rules that conflict with Prettier, and the v10 rule updates do not change its placement requirement .

export default defineConfig(
  js.configs.recommended,
  tseslint.configs.recommended,
  eslintConfigPrettier, // last
);

The Migration Command

The @eslint/migrate-config package converts a legacy .eslintrc into a starting flat config :

npx @eslint/migrate-config .eslintrc.json

The output is a starting point, not a finished config. Review the imports, the globals, and the plugin objects by hand.

The One-Line Summary

Flat only. Node 20.19+. Per-file config lookup. import-x over import. eslint-config-prettier last.

ESLint v10 is the cleanup release that removes the compatibility layers. The flat config is the only config, the Node.js floor is the gate, and the eslint-plugin-import swap is the ecosystem blocker. The rule updates produce new reports, and the eslint-env comments are errors. The config is an array of objects, and the defineConfig helper is the composition mechanism.

Summary

Area	Key takeaway
Node.js	`^20.19.0 \|\| ^22.13.0 \|\| >=24`
Config	`eslint.config.js` only
Lookup	From each file’s directory
`eslint:recommended`	Updated with new rules
`radix`	Radix always required
`globalThis`	Reported by `no-shadow-restricted-names`
`eslint-env`	Reported as errors
`eslint-plugin-import`	Swap to `eslint-plugin-import-x`
Prettier	`eslint-config-prettier` last

Remember: ESLint v10 is flat-config only and Node 20.19+ only. The config lookup starts from each file’s directory, the eslint:recommended config includes new rules, and the eslint-env comments are errors. The eslint-plugin-import package is the ecosystem blocker, and eslint-plugin-import-x is the drop-in replacement. The defineConfig and globalIgnores helpers are the composition mechanism, and eslint-config-prettier stays last. The cheatsheet is the decisions, and the decisions are the migration.

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