npm init @eslint/config@latest

npm install --save-dev eslint@latest @eslint/js@latest

import { defineConfig } from "eslint/config";
import js from "@eslint/js";

export default defineConfig([
  {
    files: ["**/*.js"],
    plugins: { js },
    extends: ["js/recommended"],
    rules: {
      "no-unused-vars": "warn",
      "no-undef": "error",
    },
  },
]);

npx eslint src/

// Apply rules only to source files
{ files: ["src/**/*.{js,ts}"], rules: { ... } }

// Apply rules to all except tests
{ files: ["**/*.js"], ignores: ["**/*.test.js"], rules: { ... } }

import { defineConfig, globalIgnores } from "eslint/config";

export default defineConfig([
  globalIgnores(["dist/", "coverage/", "*.min.js"]),
  // other config objects...
]);

import { includeIgnoreFile } from "@eslint/compat";
import { fileURLToPath } from "node:url";

const gitignorePath = fileURLToPath(new URL(".gitignore", import.meta.url));

export default defineConfig([
  includeIgnoreFile(gitignorePath),
  // ...
]);

rules: {
  "no-var": "error",          // always use const/let
  "prefer-const": "error",    // enforce immutability where possible
  "eqeqeq": ["error", "always"], // ban == in favor of ===
  "no-console": "warn",       // flag but don't block during development
}

// eslint-disable-next-line no-console -- CLI entrypoint needs output
console.log("Starting server...");

linterOptions: {
  reportUnusedDisableDirectives: "error",
}

{
  files: ["**/*.test.{js,ts}", "**/*.spec.{js,ts}"],
  rules: {
    "no-unused-expressions": "off", // chai/jest assertions
  },
}

import jsdoc from "eslint-plugin-jsdoc";
import { defineConfig } from "eslint/config";

export default defineConfig([
  {
    files: ["**/*.js"],
    plugins: { jsdoc },
    rules: {
      "jsdoc/require-description": "error",
      "jsdoc/check-values": "error",
    },
  },
]);

import react from "eslint-plugin-react";
import { defineConfig } from "eslint/config";

export default defineConfig([
  {
    files: ["**/*.{jsx,tsx}"],
    plugins: { react },
    extends: ["react/recommended"],
    languageOptions: {
      parserOptions: { ecmaFeatures: { jsx: true } },
    },
  },
]);

npm install --save-dev typescript-eslint

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

export default defineConfig([
  ...tseslint.configs.recommended,
  {
    files: ["**/*.ts", "**/*.tsx"],
    rules: {
      "@typescript-eslint/no-explicit-any": "warn",
      "@typescript-eslint/explicit-function-return-type": "off",
    },
  },
]);

{
  linterOptions: {
    noInlineConfig: false,                    // allow inline eslint comments
    reportUnusedDisableDirectives: "error",   // fail on stale disables
    reportUnusedInlineConfigs: "warn",        // warn on redundant inline configs
  },
}

npx eslint --inspect-config        # Opens config inspector UI
npx eslint --print-config file.js  # Prints merged config for a file

{
  "scripts": {
    "lint": "eslint src/",
    "lint:fix": "eslint src/ --fix"
  }
}