Switch from ESLint 8.x to 9.x

With ESLint 8.x being deprecated next month, and with the automatic
migration tool not having changed its output much in the 3 months it's
been available, I think it's time to finish out this effort that I
started back in June (and rebased onto main with just a few fixups).

I'm still not convinced that I prefer the flat config to the original
config, but I think I've put it into a state I can be happy with.

client/eslint.config.mjs

@@ -0,0 +1,65 @@
+// @ts-check
+
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+import confusingBrowserGlobals from "confusing-browser-globals";
+
+import js from "@eslint/js";
+import { fixupConfigRules, fixupPluginRules } from "@eslint/compat";
+import { FlatCompat } from "@eslint/eslintrc";
+
+import react from "eslint-plugin-react";
+import reactHooks from "eslint-plugin-react-hooks";
+import jsxA11Y from "eslint-plugin-jsx-a11y";
+import tseslint from "typescript-eslint";
+
+const compat = new FlatCompat({
+  baseDirectory: path.dirname(fileURLToPath(import.meta.url)),
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all,
+});
+
+export default tseslint.config(
+  {
+    ignores: ["dist/**/*"],
+  },
+
+  // @ts-ignore
+  ...fixupConfigRules(
+    compat.extends(
+      "eslint:recommended",
+      "plugin:react/recommended",
+      "plugin:react-hooks/recommended",
+      "plugin:jsx-a11y/recommended",
+    ),
+  ),
+  {
+    plugins: {
+      // @ts-ignore
+      react: fixupPluginRules(react),
+      // @ts-ignore
+      "react-hooks": fixupPluginRules(reactHooks),
+      "jsx-a11y": fixupPluginRules(jsxA11Y),
+    },
+    settings: {
+      react: {
+        version: "detect",
+      },
+    },
+    rules: {
+      "no-restricted-globals": ["error", ...confusingBrowserGlobals],
+    },
+  },
+
+  ...tseslint.configs.recommended.map((config) => ({
+    files: ["**/*.ts?(x)"],
+    ...config,
+  })),
+  {
+    files: ["**/*.ts?(x)"],
+    rules: {
+      "@typescript-eslint/no-explicit-any": "off",
+    },
+  },
+);

client/package.json

@@ -4,15 +4,16 @@
   "private": true,
   "type": "module",
   "devDependencies": {
+    "@eslint/compat": "^1.1.1",
+    "@eslint/eslintrc": "^3.1.0",
+    "@eslint/js": "^9.9.1",
     "@types/node": "^22.1.0",
     "@types/react": "^18.0.10",
     "@types/react-dom": "^18.0.5",
     "@types/react-helmet": "^6.1.0",
-    "@typescript-eslint/eslint-plugin": "^7.0.1",
-    "@typescript-eslint/parser": "^7.0.1",
     "@vitejs/plugin-react": "^4.0.0",
     "confusing-browser-globals": "^1.0.11",
-    "eslint": "^8.36.0",
+    "eslint": "^9.9.1",
     "eslint-plugin-jsx-a11y": "^6.7.1",
     "eslint-plugin-react": "^7.32.2",
     "eslint-plugin-react-hooks": "^4.6.0",
@@ -23,6 +24,7 @@
     "react-helmet": "^6.1.0",
     "sass": "^1.30.0",
     "typescript": "^5.0.2",
+    "typescript-eslint": "^8.3.0",
     "vite": "^5.0.0"
   },
   "scripts": {

client/src/WebRTC/Backend.ts

@@ -6,10 +6,6 @@ export type ConnectionState =
 
 type Message = { SDP: any };
 
-/* eslint-disable @typescript-eslint/adjacent-overload-signatures */
-// I find it clearer and more maintainable to keep the signatures for a single
-// event next to each other.
-
 /* eslint-disable @typescript-eslint/no-unsafe-declaration-merging */
 // TODO: I need to figure out what's up with this one.