fix: ensure bindings always take precedence over spreads (#14575)

.changeset/little-berries-worry.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure bindings always take precedence over spreads

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js

@@ -20,6 +20,8 @@ import { determine_slot } from '../../../../../utils/slot.js';
 export function build_component(node, component_name, context, anchor = context.state.node) {
 	/** @type {Array<Property[] | Expression>} */
 	const props_and_spreads = [];
+	/** @type {Array<() => void>} */
+	const delayed_props = [];
 
 	/** @type {ExpressionStatement[]} */
 	const lets = [];
@@ -63,14 +65,23 @@ export function build_component(node, component_name, context, anchor = context.
 
 	/**
 	 * @param {Property} prop
+	 * @param {boolean} [delay]
 	 */
-	function push_prop(prop) {
-		const current = props_and_spreads.at(-1);
-		const current_is_props = Array.isArray(current);
-		const props = current_is_props ? current : [];
-		props.push(prop);
-		if (!current_is_props) {
-			props_and_spreads.push(props);
+	function push_prop(prop, delay = false) {
+		const do_push = () => {
+			const current = props_and_spreads.at(-1);
+			const current_is_props = Array.isArray(current);
+			const props = current_is_props ? current : [];
+			props.push(prop);
+			if (!current_is_props) {
+				props_and_spreads.push(props);
+			}
+		};
+
+		if (delay) {
+			delayed_props.push(do_push);
+		} else {
+			do_push();
 		}
 	}
 
@@ -202,22 +213,27 @@ export function build_component(node, component_name, context, anchor = context.
 					attribute.expression.type === 'Identifier' &&
 					context.state.scope.get(attribute.expression.name)?.kind === 'store_sub';
 
+				// Delay prop pushes so bindings come at the end, to avoid spreads overwriting them
 				if (is_store_sub) {
 					push_prop(
-						b.get(attribute.name, [b.stmt(b.call('$.mark_store_binding')), b.return(expression)])
+						b.get(attribute.name, [b.stmt(b.call('$.mark_store_binding')), b.return(expression)]),
+						true
 					);
 				} else {
-					push_prop(b.get(attribute.name, [b.return(expression)]));
+					push_prop(b.get(attribute.name, [b.return(expression)]), true);
 				}
 
 				const assignment = b.assignment('=', attribute.expression, b.id('$$value'));
 				push_prop(
-					b.set(attribute.name, [b.stmt(/** @type {Expression} */ (context.visit(assignment)))])
+					b.set(attribute.name, [b.stmt(/** @type {Expression} */ (context.visit(assignment)))]),
+					true
 				);
 			}
 		}
 	}
 
+	delayed_props.forEach((fn) => fn());
+
 	if (slot_scope_applies_to_itself) {
 		context.state.init.push(...lets);
 	}

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/component.js

@@ -13,6 +13,8 @@ import { is_element_node } from '../../../../nodes.js';
 export function build_inline_component(node, expression, context) {
 	/** @type {Array<Property[] | Expression>} */
 	const props_and_spreads = [];
+	/** @type {Array<() => void>} */
+	const delayed_props = [];
 
 	/** @type {Property[]} */
 	const custom_css_props = [];
@@ -49,14 +51,23 @@ export function build_inline_component(node, expression, context) {
 
 	/**
 	 * @param {Property} prop
+	 * @param {boolean} [delay]
 	 */
-	function push_prop(prop) {
-		const current = props_and_spreads.at(-1);
-		const current_is_props = Array.isArray(current);
-		const props = current_is_props ? current : [];
-		props.push(prop);
-		if (!current_is_props) {
-			props_and_spreads.push(props);
+	function push_prop(prop, delay = false) {
+		const do_push = () => {
+			const current = props_and_spreads.at(-1);
+			const current_is_props = Array.isArray(current);
+			const props = current_is_props ? current : [];
+			props.push(prop);
+			if (!current_is_props) {
+				props_and_spreads.push(props);
+			}
+		};
+
+		if (delay) {
+			delayed_props.push(do_push);
+		} else {
+			do_push();
 		}
 	}
 
@@ -81,11 +92,12 @@ export function build_inline_component(node, expression, context) {
 			const value = build_attribute_value(attribute.value, context, false, true);
 			push_prop(b.prop('init', b.key(attribute.name), value));
 		} else if (attribute.type === 'BindDirective' && attribute.name !== 'this') {
-			// TODO this needs to turn the whole thing into a while loop because the binding could be mutated eagerly in the child
+			// Delay prop pushes so bindings come at the end, to avoid spreads overwriting them
 			push_prop(
 				b.get(attribute.name, [
 					b.return(/** @type {Expression} */ (context.visit(attribute.expression)))
-				])
+				]),
+				true
 			);
 			push_prop(
 				b.set(attribute.name, [
@@ -95,11 +107,14 @@ export function build_inline_component(node, expression, context) {
 						)
 					),
 					b.stmt(b.assignment('=', b.id('$$settled'), b.false))
-				])
+				]),
+				true
 			);
 		}
 	}
 
+	delayed_props.forEach((fn) => fn());
+
 	/** @type {Statement[]} */
 	const snippet_declarations = [];
 

packages/svelte/tests/runtime-runes/samples/bind-and-spread-precedence/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	ssrHtml: `<input value="foo">`,
+
+	test({ assert, target }) {
+		assert.equal(target.querySelector('input')?.value, 'foo');
+	}
+});

packages/svelte/tests/runtime-runes/samples/bind-and-spread-precedence/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	let { value = $bindable(), ...properties } = $props();
+</script>
+
+<input bind:value {...properties} />

packages/svelte/tests/runtime-runes/samples/bind-and-spread-precedence/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Button from './input.svelte';
+	
+	let value = $state('foo');
+
+	const props = $state({
+		value: 'bar'
+	});
+</script>
+
+<Button bind:value {...props} />