feat(prefer-svelte-reactivity): ignoring variables encapsulated in functions

Author: marekdedic

.changeset/long-ghosts-prove.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-svelte': minor
+---
+
+feat(prefer-svelte-reactivity): ignoring variables encapsulated in functions

docs/rules/prefer-svelte-reactivity.md

@@ -105,7 +105,18 @@ export default e;
 
 ## :wrench: Options
 
-Nothing.
+```json
+{
+  "svelte/prefer-svelte-reactivity": [
+    "error",
+    {
+      "ignoreEncapsulatedLocalVariables": true
+    }
+  ]
+}
+```
+
+- `ignoreEncapsulatedLocalVariables` ... Whether to ignore variables that are defined inside a function and aren't returned, thus being encapsulated in the function. Default `true`.
 
 ## :books: Further Reading
 

packages/eslint-plugin-svelte/src/rule-types.ts

@@ -320,7 +320,7 @@ export interface RuleOptions {
    * disallow using mutable instances of built-in classes where a reactive alternative is provided by svelte/reactivity
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-svelte-reactivity/
    */
-  'svelte/prefer-svelte-reactivity'?: Linter.RuleEntry<[]>
+  'svelte/prefer-svelte-reactivity'?: Linter.RuleEntry<SveltePreferSvelteReactivity>
   /**
    * Prefer using writable $derived instead of $state and $effect
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-writable-derived/
@@ -580,6 +580,10 @@ type SveltePreferConst = []|[{
   excludedRunes?: string[]
   [k: string]: unknown | undefined
 }]
+// ----- svelte/prefer-svelte-reactivity -----
+type SveltePreferSvelteReactivity = []|[{
+  ignoreEncapsulatedLocalVariables?: boolean
+}]
 // ----- svelte/require-event-prefix -----
 type SvelteRequireEventPrefix = []|[{
   checkAsyncFunctions?: boolean

packages/eslint-plugin-svelte/src/rules/prefer-svelte-reactivity.ts

@@ -12,7 +12,18 @@ export default createRule('prefer-svelte-reactivity', {
 			category: 'Possible Errors',
 			recommended: true
 		},
-		schema: [],
+		schema: [
+			{
+				type: 'object',
+				properties: {
+					ignoreEncapsulatedLocalVariables: {
+						type: 'boolean',
+						default: true
+					}
+				},
+				additionalProperties: false
+			}
+		],
 		messages: {
 			mutableDateUsed:
 				'Found a mutable instance of the built-in Date class. Use SvelteDate instead.',
@@ -31,6 +42,8 @@ export default createRule('prefer-svelte-reactivity', {
 		]
 	},
 	create(context) {
+		const ignoreEncapsulatedLocalVariables =
+			context.options[0]?.ignoreEncapsulatedLocalVariables ?? true;
 		const returnedVariables: Map<
 			TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration,
 			TSESTree.VariableDeclarator[]
@@ -139,6 +152,9 @@ export default createRule('prefer-svelte-reactivity', {
 							node
 						});
 					}
+					if (ignoreEncapsulatedLocalVariables && isLocalVarEncapsulated(returnedVariables, node)) {
+						continue;
+					}
 					if (path[0] === 'Date' && isDateMutable(referenceTracker, node as TSESTree.Expression)) {
 						context.report({
 							messageId: 'mutableDateUsed',
@@ -205,6 +221,22 @@ function findEnclosingReturn(node: TSESTree.Node): TSESTree.ReturnStatement | nu
 	return findAncestorOfTypes(node, ['ReturnStatement']);
 }
 
+function isLocalVarEncapsulated(
+	returnedVariables: Map<
+		TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration,
+		TSESTree.VariableDeclarator[]
+	>,
+	node: TSESTree.Node
+): boolean {
+	const enclosingFunction = findEnclosingFunction(node);
+	if (enclosingFunction === null) {
+		return false;
+	}
+	return (
+		returnedVariables.get(enclosingFunction)?.some((variable) => isIn(node, variable)) !== true
+	);
+}
+
 function isDateMutable(referenceTracker: ReferenceTracker, ctorNode: TSESTree.Expression): boolean {
 	return !referenceTracker
 		.iteratePropertyReferences(ctorNode, {