feat: add autofixer to tell the LLM to check if some function called in effect is assigning state

Author: paoloricciuti

packages/mcp-server/src/mcp/autofixers/add-autofixers-issues.test.ts

@@ -117,6 +117,36 @@ describe('add_autofixers_issues', () => {
 				);
 			});
 		});
+
+		it('should add a suggestion when calling a function inside an effect', () => {
+			const content = run_autofixers_on_code(`
+			<script>
+				import { fetch_data } from './data.js';
+				$effect(() => {
+					fetch_data();
+				});
+			</script>`);
+
+			expect(content.suggestions.length).toBeGreaterThanOrEqual(1);
+			expect(content.suggestions).toContain(
+				`You are calling the function \`fetch_data\` inside an $effect. Please check if the function is reassigning a stateful variable because that's considered malpractice and check if it could use  \`$derived\` instead. Ignore this suggestion if you are sure this function is not assigning any stateful variable or if you can't check if it does.`,
+			);
+		});
+
+		it('should add a suggestion when calling a function inside an effect (with non identifier callee)', () => {
+			const content = run_autofixers_on_code(`
+			<script>
+				import { fetch_data } from './data.js';
+				$effect(() => {
+					fetch_data.fetch();
+				});
+			</script>`);
+
+			expect(content.suggestions.length).toBeGreaterThanOrEqual(1);
+			expect(content.suggestions).toContain(
+				`You are calling a function inside an $effect. Please check if the function is reassigning a stateful variable because that's considered malpractice and check if it could use  \`$derived\` instead. Ignore this suggestion if you are sure this function is not assigning any stateful variable or if you can't check if it does.`,
+			);
+		});
 	});
 
 	with_possible_inits('($init)', ({ init }) => {

packages/mcp-server/src/mcp/autofixers/visitors/assign-in-effect.ts

@@ -1,4 +1,10 @@
-import type { AssignmentExpression, Identifier, Node, UpdateExpression } from 'estree';
+import type {
+	AssignmentExpression,
+	CallExpression,
+	Identifier,
+	Node,
+	UpdateExpression,
+} from 'estree';
 import type { Autofixer, AutofixerState } from './index.js';
 import { left_most_id } from '../ast/utils.js';
 import type { AST } from 'svelte-eslint-parser';
@@ -27,9 +33,9 @@ function run_if_in_effect(
 	}
 }
 
-function visitor(
+function assign_or_update_visitor(
 	node: UpdateExpression | AssignmentExpression,
-	{ state, path }: Context<Node | AST.SvelteNode, AutofixerState>,
+	{ state, path, next }: Context<Node | AST.SvelteNode, AutofixerState>,
 ) {
 	run_if_in_effect(path, state, () => {
 		function check_if_stateful_id(id: Identifier) {
@@ -58,9 +64,25 @@ function visitor(
 			}
 		}
 	});
+	next();
+}
+
+function call_expression_visitor(
+	node: CallExpression,
+	{ state, path, next }: Context<Node | AST.SvelteNode, AutofixerState>,
+) {
+	run_if_in_effect(path, state, () => {
+		const function_name =
+			node.callee.type === 'Identifier' ? `the function \`${node.callee.name}\`` : 'a function';
+		state.output.suggestions.push(
+			`You are calling ${function_name} inside an $effect. Please check if the function is reassigning a stateful variable because that's considered malpractice and check if it could use  \`$derived\` instead. Ignore this suggestion if you are sure this function is not assigning any stateful variable or if you can't check if it does.`,
+		);
+	});
+	next();
 }
 
 export const assign_in_effect: Autofixer = {
-	UpdateExpression: visitor,
-	AssignmentExpression: visitor,
+	UpdateExpression: assign_or_update_visitor,
+	AssignmentExpression: assign_or_update_visitor,
+	CallExpression: call_expression_visitor,
 };