fix mutation to imported variable

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Fix reactivity with imported values that are then mutated ([#4555](https://github.com/sveltejs/svelte/issues/4555))
+
 ## 3.21.0
 
 * Support dimension bindings in cross-origin environments ([#2147](https://github.com/sveltejs/svelte/issues/2147))

src/compiler/compile/Component.ts

@@ -632,7 +632,6 @@ export default class Component {
 			this.add_var({
 				name,
 				initialised: instance_scope.initialised_declarations.has(name),
-				hoistable: /^Import/.test(node.type),
 				writable
 			});
 
@@ -986,6 +985,7 @@ export default class Component {
 			hoistable_nodes,
 			var_lookup,
 			injected_reactive_declaration_vars,
+			imports,
 		} = this;
 
 		const top_level_function_declarations = new Map();
@@ -1137,6 +1137,14 @@ export default class Component {
 				this.fully_hoisted.push(node);
 			}
 		}
+
+		for (const { specifiers } of imports) {
+			for (const specifier of specifiers) {
+				const variable = var_lookup.get(specifier.local.name);
+
+				if (!variable.mutated) variable.hoistable = true;
+			}
+		}
 	}
 
 	extract_reactive_declarations() {

test/runtime/samples/reactive-import-statement-2/_config.js

@@ -0,0 +1,3 @@
+export default {
+	html: `<p>prop value</p>`
+};

test/runtime/samples/reactive-import-statement-2/data.js

@@ -0,0 +1,3 @@
+export const obj = {
+	prop: 'prop value'
+};

test/runtime/samples/reactive-import-statement-2/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	import { obj } from './data.js';
+
+	$: prop = obj.prop;
+	obj.foo = 'a different prop';
+</script>
+
+<p>{prop}</p>

test/runtime/samples/reactive-import-statement/_config.js

@@ -0,0 +1,38 @@
+import * as path from 'path';
+
+export default {
+	html: `
+		import
+		<p>1 + 2 + 3 + 4 = 10</p>
+		local
+		<p>1 + 2 + 3 + 4 = 10</p>
+		<button>Add a number</button>
+	`,
+	before_test() {
+		delete require.cache[path.resolve(__dirname, 'data.js')];
+	},
+	async test({ assert, target, window, }) {
+		const btn = target.querySelector('button');
+		const clickEvent = new window.MouseEvent('click');
+
+		await btn.dispatchEvent(clickEvent);
+
+		assert.htmlEqual(target.innerHTML, `
+			import
+			<p>1 + 2 + 3 + 4 + 5 = 15</p>
+			local
+			<p>1 + 2 + 3 + 4 + 5 = 15</p>
+			<button>Add a number</button>
+		`);
+
+		await btn.dispatchEvent(clickEvent);
+
+		assert.htmlEqual(target.innerHTML, `
+			import
+			<p>1 + 2 + 3 + 4 + 5 + 6 = 21</p>
+			local
+			<p>1 + 2 + 3 + 4 + 5 + 6 = 21</p>
+			<button>Add a number</button>
+		`);
+	}
+};

test/runtime/samples/reactive-import-statement/data.js

@@ -0,0 +1 @@
+export const numbers = [1, 2, 3, 4];

test/runtime/samples/reactive-import-statement/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import { numbers } from './data.js';
+	const local_numbers = [1, 2, 3, 4];
+
+	function addNumber() {
+		numbers[numbers.length] = numbers.length + 1;
+		local_numbers[local_numbers.length] = local_numbers.length + 1;
+	}
+
+	$: sum = numbers.reduce((t, n) => t + n, 0);
+	$: local_sum = local_numbers.reduce((t, n) => t + n, 0);
+</script>
+
+import <p>{numbers.join(' + ')} = {sum}</p>
+local <p>{local_numbers.join(' + ')} = {local_sum}</p>
+
+<button on:click={addNumber}>
+	Add a number
+</button>