Upgrade to ESLint 9

[DylanSp]

Upgrade to ESLint 9, with its new flat config system. ESLint has a migration guide.

.eslintrc.cjs can be moved to eslint.config.js - a lot of the Typescript setup can be deleted, it's now handled automatically by typescript-eslint; see their migration instructions.

• Install @eslint/js, @types/eslint, and typescript-eslint as dev dependencies.
• Remove @typescript-eslint/parser and @typescript-eslint/eslint-plugin from package.json.

All of the plugins (react/recommended, react-hooks/recommended, react/jsx-runtime, prettier/recommended, and react-refresh) need to be upgraded to work with ESLint 9 before I can update. As of 4/27/2024, eslint-plugin-react definitely isn't - see jsx-eslint/eslint-plugin-react#3699. I haven't checked the other plugins.

The new file will probably look something like:

import eslint from "@eslint/js";
import tseslint from "typescript-eslint";
import eslintReact from "eslint-plugin-react";

export default tseslint.config(
  // replaces the ignorePatterns key
  {
    ignores: ["dist"],
  },

  eslint.configs.recommended, // sets up ESLint's recommended config,
  ...tseslint.configs.recommendedTypeChecked, // sets up typescript-eslint's requirements and it's "recommended-type-checked" config

  // config object with customizations
  {
    // new way of importing plugins
    plugins: {
      "react": eslintReact,
      // other plugins...
    },
   
    // rule customizations stay the same
    rules: {
      "react-refresh/only-export-components": ["warn", { allowConstantExport: true }],
      "prettier/prettier": "warn",
    },
  }
);

I'm not sure if the env config, env: { browser: true, es2020: true }, is still needed; the ESLint migration guide suggests something with languageOptions.globals. This might be handled automatically by typescript-eslint - their guides don't mention setting this.

I'm also not sure what to do with the settings config:

    settings: {
      react: {
        version: "detect",
      },
    },

This ESLint blog post on the new config system suggests that putting it first in the array of config objects should work.