chore: update ESLint to v10 (#17670)

* chore: update ESLint to v10

Update eslint and related plugins/configs. Bump CI lint job to Node 24
(ESLint 10 requires ^20.19.0 || ^22.13.0 || >=24). Replace removed
Linter.FlatConfig type with Linter.Config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address new `no-useless-assignment` violations from ESLint 10

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* dedupe

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -86,7 +86,7 @@ jobs:
       - uses: pnpm/action-setup@v4
       - uses: actions/setup-node@v6
         with:
-          node-version: 18
+          node-version: 24
           cache: pnpm
       - name: install
         run: pnpm install --frozen-lockfile

eslint.config.js

@@ -33,7 +33,7 @@ const no_compiler_imports = {
 	}
 };
 
-/** @type {import('eslint').Linter.FlatConfig[]} */
+/** @type {import('eslint').Linter.Config[]} */
 export default [
 	...svelte_config,
 	{

package.json

@@ -27,22 +27,30 @@
   },
   "devDependencies": {
     "@changesets/cli": "^2.29.8",
-    "@sveltejs/eslint-config": "^8.3.3",
+    "@sveltejs/eslint-config": "^8.3.5",
     "@svitejs/changesets-changelog-github-compact": "^1.1.0",
     "@types/node": "^20.11.5",
     "@types/picomatch": "^4.0.2",
     "@vitest/coverage-v8": "^2.1.9",
-    "eslint": "^9.9.1",
+    "@eslint/js": "^10.0.0",
-    "eslint-plugin-lube": "^0.4.3",
+    "eslint": "^10.0.0",
-    "eslint-plugin-svelte": "^3.11.0",
+    "eslint-plugin-lube": "^0.5.1",
+    "eslint-plugin-svelte": "^3.15.0",
     "jsdom": "25.0.1",
     "playwright": "^1.58.0",
     "prettier": "^3.2.4",
     "prettier-plugin-svelte": "^3.4.0",
     "svelte": "workspace:^",
     "typescript": "^5.5.4",
-    "typescript-eslint": "^8.48.1",
+    "typescript-eslint": "^8.55.0",
     "v8-natives": "^1.2.5",
     "vitest": "^2.1.9"
+  },
+  "pnpm": {
+    "peerDependencyRules": {
+      "allowedVersions": {
+        "eslint": "10"
+      }
+    }
   }
 }

packages/svelte/src/compiler/phases/1-parse/utils/fuzzymatch.js

@@ -46,10 +46,11 @@ function levenshtein(str1, str2) {
 	/** @type {number[]} */
 	const current = [];
 	let prev = 0;
-	let value = 0;
 
 	for (let i = 0; i <= str2.length; i++) {
 		for (let j = 0; j <= str1.length; j++) {
+			let value;
+
 			if (i && j) {
 				if (str1.charAt(j - 1) === str2.charAt(i - 1)) {
 					value = prev;

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js

@@ -538,7 +538,7 @@ function build_attr_style(style_directives, expression, context, transform) {
 				name = name.toLowerCase();
 			}
 
-			const property = b.init(directive.name, expression);
+			const property = b.init(name, expression);
 			if (directive.modifiers.includes('important')) {
 				important_properties.push(property);
 			} else {

packages/svelte/src/internal/client/proxy.js

@@ -126,7 +126,7 @@ export function proxy(value) {
 			}
 			var s = sources.get(prop);
 			if (s === undefined) {
-				s = with_parent(() => {
+				with_parent(() => {
 					var s = source(descriptor.value, stack);
 					sources.set(prop, s);
 					if (DEV && typeof prop === 'string') {

packages/svelte/src/internal/client/reactivity/sources.js

@@ -302,6 +302,7 @@ export function update_pre(source, d = 1) {
 	var value = get(source);
 
 	// @ts-expect-error
+	// eslint-disable-next-line no-useless-assignment -- `++`/`--` used for return value, not side effect on `value`
 	return set(source, d === 1 ? ++value : --value);
 }