feat: Adding a new no-ignored-unsubscribe rule.

.changeset/hip-tips-smell.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-svelte": minor
+---
+
+feat: add new `svelte/no-ignored-unsubscribe` rule.

docs/rules/no-ignored-unsubscribe.md

@@ -0,0 +1,52 @@
+---
+pageClass: 'rule-details'
+sidebarDepth: 0
+title: 'svelte/no-ignored-unsubscribe'
+description: 'disallow ignoring the return value of `subscribe()`'
+since: 'v2.34.0'
+---
+
+# (svelte/no-ignored-unsubscribe)
+
+> disallow ignoring the return value of `subscribe()`
+
+## :book: Rule Details
+
+This rule fails if an "unsubscriber" returned by call to `subscribe()` is neither assigned to a variable or property or passed to a function.
+
+One should always unsubscribe from a store when it is no longer needed. Otherwise, the subscription will remain active and constitute a **memory leak**.
+This rule helps to find such cases by ensuring that the unsubscriber (the return value from the store's `subscribe` method) is not ignored.
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/no-ignored-unsubscribe: "error" */
+
+  import myStore from './my-stores';
+
+  /* ✓ GOOD */
+  const unsubscribe = myStore.subscribe(() => {});
+
+  /* ✗ BAD */
+  myStore.subscribe(() => {});
+
+</script>
+```
+
+</ESLintCodeBlock>
+
+## :wrench: Options
+
+Nothing.
+
+## :rocket: Version
+
+This rule was introduced in eslint-plugin-svelte v2.34.0
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/src/rules/no-ignored-unsubscribe.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/tests/src/rules/no-ignored-unsubscribe.ts)

src/rules/no-ignored-unsubscribe.ts

@@ -0,0 +1,30 @@
+import type { TSESTree } from '@typescript-eslint/types';
+import { createRule } from "../utils";
+
+export default createRule("no-ignored-unsubscribe", {
+	meta: {
+		docs: {
+			description: "Forbids ignoring the unsubscribe method returned by the `subscribe()` os Svelte stores.",
+			category: 'Best Practices',
+			recommended: false,
+		},
+		fixable: undefined,
+		hasSuggestions: false,
+		messages: {
+			forbidden: "Ignoring returned value of the subscribe method is forbidden.",
+		},
+		schema: [],
+		type: "problem",
+	},
+	create: (context) => {
+		return {
+			"ExpressionStatement > CallExpression > MemberExpression[property.name='subscribe']":
+				(node: TSESTree.MemberExpression) => {
+					context.report({
+						messageId: "forbidden",
+						node: node.property,
+					});
+				},
+		};
+	},
+});

src/utils/rules.ts

@@ -61,6 +61,7 @@ import system from '../rules/system';
 import validCompile from '../rules/valid-compile';
 import validEachKey from '../rules/valid-each-key';
 import validPropNamesInKitPages from '../rules/valid-prop-names-in-kit-pages';
+import noIgnoredUnsubscribe from "../rules/no-ignored-unsubscribe";
 
 export const rules = [
 	typescriptEslintNoUnnecessaryCondition,
@@ -88,6 +89,7 @@ export const rules = [
 	noDynamicSlotName,
 	noExportLoadInSvelteModuleInKitPages,
 	noExtraReactiveCurlies,
+	noIgnoredUnsubscribe,
 	noImmutableReactiveStatements,
 	noInnerDeclarations,
 	noNotFunctionHandler,

tests/fixtures/rules/no-ignored-unsubscribe/invalid/test01-errors.yaml

@@ -0,0 +1,4 @@
+- message: Ignoring returned value of the subscribe method is forbidden.
+  line: 6
+  column: 9
+  suggestions: null

tests/fixtures/rules/no-ignored-unsubscribe/invalid/test01-input.svelte

@@ -0,0 +1,9 @@
+<script>
+    import {writable} from "svelte/store";
+
+    const foo = writable(0);
+
+    foo.subscribe(() => {
+        console.log('foo changed');
+    });
+</script>

tests/fixtures/rules/no-ignored-unsubscribe/valid/test01-input.svelte

@@ -0,0 +1,9 @@
+<script>
+    import {writable} from "svelte/store";
+
+    const foo = writable(0);
+
+    const unsubscribe = foo.subscribe(() => {
+        console.log('foo changed');
+    });
+</script>

tests/fixtures/rules/no-ignored-unsubscribe/valid/test02-input.svelte

@@ -0,0 +1,10 @@
+<script>
+    import {writable} from "svelte/store";
+
+    const foo = writable(0);
+
+    const unsubscribers = [];
+    unsubscribers.push(foo.subscribe(() => {
+        console.log('foo changed');
+    }));
+</script>

tests/src/rules/no-ignored-unsubscribe.ts

@@ -0,0 +1,12 @@
+import { RuleTester } from 'eslint';
+import rule from '../../../src/rules/no-ignored-unsubscribe';
+import { loadTestCases } from '../../utils/utils';
+
+const tester = new RuleTester({
+	parserOptions: {
+		ecmaVersion: 2020,
+		sourceType: 'module',
+	}
+});
+
+tester.run('no-ignored-unsubscribe', rule as any, loadTestCases('no-ignored-unsubscribe'));