feat: upgrade to Next.js 15, React 19, and migrate to ESLint 9 flat config

Major framework upgrades:
- Next.js 13 → 15.5.6
- React 18.2.0 → 19.2.0
- ESLint 8 → 9 with flat config migration

Dependency updates:
- @headlessui/react, date-fns, zod, true-myth, usehooks-ts
- @typescript-eslint packages to v8
- TypeScript to v5.9.3
- Prettier to v3.6.2

Breaking changes:
- Migrate from .eslintrc.json to eslint.config.mjs
- Remove deprecated swcMinify option from next.config.mjs
- Update all React type definitions

.eslintrc.json

@@ -1,22 +0,0 @@
-{
-  "overrides": [
-    {
-      "extends": [
-        "plugin:@typescript-eslint/recommended-requiring-type-checking"
-      ],
-      "files": ["*.ts", "*.tsx"],
-      "parserOptions": {
-        "project": "tsconfig.json"
-      }
-    }
-  ],
-  "parser": "@typescript-eslint/parser",
-  "parserOptions": {
-    "project": "./tsconfig.json"
-  },
-  "plugins": ["@typescript-eslint"],
-  "extends": ["next/core-web-vitals", "plugin:@typescript-eslint/recommended"],
-  "rules": {
-    "@typescript-eslint/consistent-type-imports": "warn"
-  }
-}

eslint.config.mjs

@@ -0,0 +1,76 @@
+import typescriptEslint from "@typescript-eslint/eslint-plugin";
+import tsParser from "@typescript-eslint/parser";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all
+});
+
+export default [
+  // Base configuration with ignores
+  {
+    ignores: [
+      ".next/**",
+      "node_modules/**",
+      "out/**",
+      "*.config.mjs",
+      "*.config.js",
+      "next-env.d.ts"  // Next.js generated file
+    ],
+  },
+
+  // Next.js core web vitals using FlatCompat
+  ...compat.extends("next/core-web-vitals"),
+
+  // TypeScript recommended rules
+  ...compat.extends("plugin:@typescript-eslint/recommended"),
+
+  // Base TypeScript configuration
+  {
+    plugins: {
+      "@typescript-eslint": typescriptEslint,
+    },
+
+    languageOptions: {
+      parser: tsParser,
+      ecmaVersion: "latest",
+      sourceType: "module",
+      parserOptions: {
+        project: "./tsconfig.json",
+      },
+    },
+
+    rules: {
+      "@typescript-eslint/consistent-type-imports": "warn",
+    },
+  },
+
+  // Additional strict TypeScript rules for .ts and .tsx files
+  {
+    files: ["**/*.ts", "**/*.tsx"],
+    ...compat.extends("plugin:@typescript-eslint/recommended-requiring-type-checking")[0],
+
+    languageOptions: {
+      ecmaVersion: "latest",
+      sourceType: "module",
+      parserOptions: {
+        project: "./tsconfig.json",
+      },
+    },
+  },
+
+  // Allow CommonJS require in .cjs files
+  {
+    files: ["**/*.cjs"],
+    rules: {
+      "@typescript-eslint/no-require-imports": "off",
+    },
+  },
+];

next.config.mjs

@@ -8,7 +8,6 @@
 /** @type {import("next").NextConfig} */
 const config = {
   reactStrictMode: true,
-  swcMinify: true,
   i18n: {
     locales: ["en"],
     defaultLocale: "en",

package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "build": "next build",
     "dev": "next dev",
-    "lint": "next lint",
+    "lint": "eslint .",
     "start": "next start",
     "test": "vitest",
     "test:ui": "vitest --ui",
@@ -15,43 +15,41 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
-    "@headlessui/react": "^2.0.3",
+    "@headlessui/react": "^2.2.9",
     "@heroicons/react": "^2.0.16",
     "@swc/helpers": "^0.5.11",
     "clsx": "^2.1.1",
-    "date-fns": "^2.29.3",
+    "date-fns": "^4.1.0",
     "immutability-helper": "^3.1.1",
-    "next": "^13",
-    "react": "18.2.0",
-    "react-dom": "18.2.0",
+    "next": "^15.5.6",
+    "react": "19.2.0",
+    "react-dom": "19.2.0",
     "react-hook-form": "^7.42.1",
-    "react-timeago": "^7.2.0",
+    "react-timeago": "^8.3.0",
     "sass": "^1.57.1",
-    "true-myth": "^7.1.0",
-    "usehooks-ts": "^2.9.1",
-    "zod": "^3.20.2"
+    "true-myth": "^9.2.0",
+    "usehooks-ts": "^3.1.1",
+    "zod": "^4.1.12"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.0",
-    "@types/node": "^18.11.18",
-    "@types/prettier": "^2.7.2",
-    "@types/react": "^18.0.26",
-    "@types/react-dom": "^18.0.10",
-    "@types/react-timeago": "^4.1.7",
-    "@typescript-eslint/eslint-plugin": "^5.47.1",
-    "@typescript-eslint/parser": "^5.47.1",
+    "@types/node": "^24.9.1",
+    "@types/react": "^19.2.2",
+    "@types/react-dom": "^19.2.2",
+    "@typescript-eslint/eslint-plugin": "^8.46.2",
+    "@typescript-eslint/parser": "^8.46.2",
     "@vitest/ui": "^3.2.4",
     "autoprefixer": "^10.4.7",
-    "eslint": "^8.30.0",
-    "eslint-config-next": "13.1.1",
+    "eslint": "^9.38.0",
+    "eslint-config-next": "15.5.6",
     "happy-dom": "^20.0.8",
     "postcss": "^8.4.14",
-    "prettier": "^2.8.8",
-    "prettier-plugin-tailwindcss": "^0.2.1",
+    "prettier": "^3.6.2",
+    "prettier-plugin-tailwindcss": "^0.7.1",
     "tailwindcss": "^3.2.0",
-    "type-fest": "^4.18.2",
-    "typescript": "^4.9.4",
+    "type-fest": "^5.1.0",
+    "typescript": "^5.9.3",
     "vitest": "^3.2.4"
   },
   "ct3aMetadata": {

src/components/common/Property.tsx

@@ -1,10 +1,10 @@
-import type { FunctionComponent, ReactFragment, ReactNode } from "react";
+import type { FunctionComponent, ReactNode } from "react";
 import React from "react";
 import clsx from "clsx";
 
 type PropertyProps = {
-  title: string | ReactNode | ReactFragment;
-  children: string | ReactNode | ReactFragment;
+  title: string | ReactNode;
+  children: string | ReactNode;
   titleClass?: string;
   valueClass?: string;
 };

src/helpers.ts

@@ -134,7 +134,7 @@ export function ipv6InCIDR(ip: string, cidr: string): boolean {
     const mask = (maxMask << BigInt(128 - prefixLen)) & maxMask;
 
     return (ipInt & mask) === (rangeInt & mask);
-  } catch (e) {
+  } catch {
     return false;
   }
 }

src/hooks/useLookup.tsx

@@ -212,7 +212,7 @@ const useLookup = (warningHandler?: WarningHandler) => {
 
   async function getAndParse<T>(
     url: string,
-    schema: ZodSchema
+    schema: ZodSchema<T>
   ): Promise<Result<T, Error>> {
     const response = await fetch(url);
 
@@ -366,7 +366,6 @@ const useLookup = (warningHandler?: WarningHandler) => {
             )
           );
 
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
         const data = await response.json();
 
         // Try each schema until one works
@@ -381,7 +380,6 @@ const useLookup = (warningHandler?: WarningHandler) => {
         );
       }
       case "json": {
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
         const data = JSON.parse(target);
         for (const schema of schemas) {
           const result = schema.safeParse(data);

src/rdap.ts

@@ -796,7 +796,7 @@ const TypeValidators = new Map<
         const octet = parseInt(octets[i] ?? "", 10);
         if (isNaN(octet) || octet < 0 || octet > 255) {
           return Promise.resolve(
-            `Invalid IPv4 address: octet ${i + 1} (${octets[i]}) must be 0-255`
+            `Invalid IPv4 address: octet ${i + 1} (${octets[i] ?? 'undefined'}) must be 0-255`
           );
         }
       }