chore: upgrade eslint to v9 (#11108)

Author: jrmajor

.editorconfig

@@ -11,5 +11,5 @@ trim_trailing_whitespace = true
 [test/**/expected.css]
 insert_final_newline = false
 
-[{package.json,.travis.yml,.eslintrc.json}]
+[package.json]
 indent_style = space

.eslintignore

@@ -1,5 +1,3 @@
-# NOTE: In general this should be kept in sync with .eslintignore
-
 packages/**/dist/*.js
 packages/**/build/*.js
 packages/**/npm/**/*
@@ -31,7 +29,6 @@ sites/svelte.dev/src/lib/generated
 **/.vercel
 .github/CODEOWNERS
 .prettierignore
-.eslintignore
 .changeset
 pnpm-lock.yaml
 pnpm-workspace.yaml

.eslintrc.cjs

@@ -0,0 +1,45 @@
+import svelte_config from '@sveltejs/eslint-config';
+import lube from 'eslint-plugin-lube';
+
+/** @type {import('eslint').Linter.FlatConfig[]} */
+export default [
+	...svelte_config,
+	{
+		plugins: {
+			lube
+		},
+		rules: {
+			'no-console': 'error',
+			'lube/svelte-naming-convention': ['error', { fixSameNames: true }],
+			// eslint isn't that well-versed with JSDoc to know that `foo: /** @type{..} */ (foo)` isn't a violation of this rule, so turn it off
+			'object-shorthand': 'off',
+			'no-var': 'off',
+
+			// TODO: enable these rules and run `pnpm lint:fix`
+			// skipping that for now so as to avoid impacting real work
+			'@stylistic/quotes': 'off',
+			'@typescript-eslint/no-unused-vars': 'off',
+			'prefer-const': 'off'
+		}
+	},
+	{
+		files: ['playgrounds/**/*'],
+		rules: {
+			'lube/svelte-naming-convention': 'off',
+			'no-console': 'off'
+		}
+	},
+	{
+		ignores: [
+			'**/*.d.ts',
+			'**/tests',
+			'packages/svelte/compiler/index.js',
+			// documentation can contain invalid examples
+			'documentation',
+			// contains a fork of the REPL which doesn't adhere to eslint rules
+			'sites/svelte-5-preview/**',
+			// wasn't checked previously, reenable at some point
+			'sites/svelte.dev/**'
+		]
+	}
+];

.prettierignore

@@ -19,28 +19,28 @@
     "preview-site": "npm run build --prefix sites/svelte-5-preview",
     "check": "cd packages/svelte && pnpm build && cd ../../ && pnpm -r check",
     "format": "prettier --write .",
-    "lint": "prettier --check . && eslint ./",
+    "lint": "prettier --check . && eslint",
     "test": "vitest run",
     "test-output": "vitest run --coverage --reporter=json --outputFile=sites/svelte-5-preview/src/routes/status/results.json",
     "changeset:version": "changeset version && pnpm -r generate:version && git add --all",
     "changeset:publish": "changeset publish"
   },
   "devDependencies": {
     "@changesets/cli": "^2.27.1",
-    "@sveltejs/eslint-config": "^6.0.4",
+    "@sveltejs/eslint-config": "^7.0.1",
     "@svitejs/changesets-changelog-github-compact": "^1.1.0",
     "@types/node": "^20.11.5",
-    "@typescript-eslint/eslint-plugin": "^6.21.0",
     "@vitest/coverage-v8": "^1.2.1",
     "concurrently": "^8.2.2",
     "cross-env": "^7.0.3",
-    "eslint": "^8.56.0",
+    "eslint": "^9.0.0",
     "eslint-plugin-lube": "^0.4.3",
     "jsdom": "22.0.0",
     "playwright": "^1.41.1",
     "prettier": "^3.2.4",
     "prettier-plugin-svelte": "^3.1.2",
     "typescript": "^5.3.3",
+    "typescript-eslint": "^7.6.0",
     "vitest": "^1.2.1"
   },
   "pnpm": {

eslint.config.js

@@ -131,7 +131,6 @@ export function bind_checked(input, get_value, update) {
 		update(value);
 	});
 
-	// eslint-disable-next-line eqeqeq
 	if (get_value() == undefined) {
 		update(false);
 	}

package.json

@@ -9,10 +9,8 @@ export function equals(value) {
  * @returns {boolean}
  */
 export function safe_not_equal(a, b) {
-	// eslint-disable-next-line eqeqeq
 	return a != a
-		? // eslint-disable-next-line eqeqeq
-			b == b
+		? b == b
 		: a !== b || (a !== null && typeof a === 'object') || typeof a === 'function';
 }
 

packages/svelte/src/internal/client/dom/elements/bindings/input.js

@@ -1,4 +1,3 @@
-// eslint-disable-next-line @typescript-eslint/no-empty-function
 export const noop = () => {};
 
 // Adapted from https://github.com/then/is-promise/blob/master/index.js

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

@@ -27,11 +27,7 @@ export function readable(value, start) {
  * @returns {boolean}
  */
 export function safe_not_equal(a, b) {
-	// eslint-disable-next-line eqeqeq
-	return a != a
-		? // eslint-disable-next-line eqeqeq
-			b == b
-		: a !== b || (a && typeof a === 'object') || typeof a === 'function';
+	return a != a ? b == b : a !== b || (a && typeof a === 'object') || typeof a === 'function';
 }
 
 /**