ESLint: Migrate to flat config

.eslintignore

@@ -1,15 +0,0 @@
-.DS_Store
-node_modules
-/build
-/.svelte-kit
-/package
-.env
-.env.*
-!.env.example
-
-/doc
-
-# Ignore files for PNPM, NPM and YARN
-pnpm-lock.yaml
-package-lock.json
-yarn.lock

.eslintrc.cjs

@@ -1,27 +0,0 @@
-module.exports = {
-	root: true,
-	parser: "@typescript-eslint/parser",
-	extends: ["plugin:svelte/recommended"],
-	plugins: ["@typescript-eslint"],
-	ignorePatterns: ["*.cjs", "*.config.js"],
-	overrides: [
-		{
-			files: ["*.svelte"],
-			parser: "svelte-eslint-parser",
-			parserOptions: {
-				parser: "@typescript-eslint/parser",
-			},
-		},
-	],
-	parserOptions: {
-		project: "./tsconfig.json",
-		extraFileExtensions: [".svelte"],
-		sourceType: "module",
-		ecmaVersion: 2020,
-	},
-	env: {
-		browser: true,
-		es2017: true,
-		node: false,
-	},
-};

CHANGELOG.md

@@ -18,6 +18,16 @@ These changes are awaiting release:
 - Workflows: Upgrade action versions & set Node version to `24`.
 - Dockerfile: Upgrade node steps to use version `24`.
 - Workflows: test-pr-server: Set go-version-file for setup-go action.
+- Upgrade devDependencies:
+  - eslint: 8.57.0 -> 10.7.0
+  - eslint-config-prettier: 10.1.2 -> 10.1.8
+  - eslint-plugin-svelte: 2.45.1 -> 3.20.0
+  - prettier: 3.4.2 -> 3.9.5
+  - prettier-plugin-svelte: 3.4.0 -> 4.1.1
+  - svelte-eslint-parser: 0.42.0 -> 1.8.0
+  - typescript-eslint: 8.32.1 -> 8.63.0
+- Added devDependencies: `@eslint/js` (new eslint has split this module out into a new package) and `globals` (as a result of new eslint architecture, we now have this as a direct dev dependency).
+- ESLint: Migrate to flat config: I ran `npx sv add eslint` and modified the eslint.config.js it generated to line up better with our old one and work a bit better (i think) with our normal ts files. Also set `ecmaVersion` to `latest`, previously was `2020`.
 
 # [4.1.1] - 2026-07-26T02:00:00Z
 

eslint.config.js

@@ -0,0 +1,60 @@
+import prettier from "eslint-config-prettier";
+import path from "node:path";
+import js from "@eslint/js";
+import svelte from "eslint-plugin-svelte";
+import { defineConfig, includeIgnoreFile } from "eslint/config";
+import globals from "globals";
+import ts from "typescript-eslint";
+import svelteConfig from "./svelte.config.js";
+
+const gitignorePath = path.resolve(import.meta.dirname, ".gitignore");
+
+export default defineConfig(
+	includeIgnoreFile(gitignorePath),
+	js.configs.recommended,
+	ts.configs.recommended,
+	svelte.configs.recommended,
+	prettier,
+	svelte.configs.prettier,
+	{
+		languageOptions: {
+			globals: { ...globals.browser /* ...globals.node */ },
+			parserOptions: {
+				projectService: true,
+				parser: ts.parser,
+				ecmaVersion: "latest",
+			},
+		},
+		rules: {
+			// typescript-eslint strongly recommend that you do not use the no-undef lint rule on TypeScript projects.
+			// see: https://typescript-eslint.io/troubleshooting/faqs/eslint/#i-get-errors-from-the-no-undef-rule-about-global-variables-not-being-defined-even-though-there-are-no-typescript-errors
+			"no-undef": "off",
+		},
+	},
+	{
+		files: ["**/*.svelte", "**/*.svelte.ts", "**/*.svelte.js"],
+		languageOptions: {
+			parserOptions: {
+				extraFileExtensions: [".svelte"],
+				svelteConfig,
+			},
+		},
+	},
+	{
+		// Override or add rule settings here.
+		rules: {
+			"@typescript-eslint/no-empty-object-type": [
+				"error",
+				{
+					// Allowing this because it's nice to create an empty
+					// interface that extends a base interface for specific
+					// use, even if it is currently empty, incase i add stuff
+					// to it in the future. It's also less confusing to read
+					// the actual type name I want instead of the base type
+					// everywhere in certain scenarios.
+					allowInterfaces: "with-single-extends",
+				},
+			],
+		},
+	},
+);

package.json

@@ -14,23 +14,24 @@
 		"format": "prettier --write ."
 	},
 	"devDependencies": {
+		"@eslint/js": "10.0.1",
 		"@sveltejs/adapter-node": "5.2.12",
 		"@sveltejs/kit": "2.21.0",
 		"@types/papaparse": "5.3.15",
-		"@typescript-eslint/eslint-plugin": "8.32.1",
-		"@typescript-eslint/parser": "8.32.1",
 		"@vite-pwa/sveltekit": "0.6.6",
-		"eslint": "8.57.0",
+		"eslint": "10.7.0",
-		"eslint-config-prettier": "10.1.2",
+		"eslint-config-prettier": "10.1.8",
-		"eslint-plugin-svelte": "2.45.1",
+		"eslint-plugin-svelte": "3.20.0",
-		"prettier": "3.4.2",
+		"globals": "17.7.0",
-		"prettier-plugin-svelte": "3.4.0",
+		"prettier": "3.9.5",
+		"prettier-plugin-svelte": "4.1.1",
 		"sass": "1.97.3",
 		"svelte": "5.17.3",
 		"svelte-check": "4.1.3",
-		"svelte-eslint-parser": "0.42.0",
+		"svelte-eslint-parser": "1.8.0",
 		"svelte-preprocess": "6.0.3",
 		"typescript": "5.8.3",
+		"typescript-eslint": "8.63.0",
 		"vite": "6.3.5"
 	},
 	"type": "module",