alternative approach to mutating props

Author: Rich-Harris

packages/svelte/src/compiler/phases/3-transform/client/utils.js

@@ -470,12 +470,14 @@ export function serialize_set_binding(node, context, fallback, options) {
 				if (binding.kind === 'prop') {
 					return b.call(
 						left,
-						b.assignment(
-							node.operator,
-							/** @type {import('estree').Pattern} */ (visit(node.left)),
-							value
-						),
-						b.literal(true)
+						b.sequence([
+							b.assignment(
+								node.operator,
+								/** @type {import('estree').Pattern} */ (visit(node.left)),
+								value
+							),
+							b.call(left)
+						])
 					);
 				} else {
 					return b.call(

packages/svelte/src/internal/client/reactivity/props.js

@@ -175,13 +175,10 @@ export function prop(props, key, flags, initial) {
 	// intermediate mode — prop is written to, but the parent component had
 	// `bind:foo` which means we can just call `$$props.foo = value` directly
 	if (setter) {
-		return function (/** @type {V} */ value, mutation = false) {
+		return function (/** @type {V} */ value) {
 			if (arguments.length === 1) {
 				/** @type {Function} */ (setter)(value);
 				return value;
-			} else if (mutation) {
-				/** @type {Function} */ (setter)(getter());
-				return value;
 			} else {
 				return getter();
 			}
@@ -213,7 +210,7 @@ export function prop(props, key, flags, initial) {
 
 	if (!immutable) current_value.equals = safe_equals;
 
-	return function (/** @type {V} */ value, mutation = false) {
+	return function (/** @type {V} */ value) {
 		var current = get(current_value);
 
 		// legacy nonsense — need to ensure the source is invalidated when necessary
@@ -229,9 +226,9 @@ export function prop(props, key, flags, initial) {
 		}
 
 		if (arguments.length > 0) {
-			if (mutation || (immutable ? value !== current : safe_not_equal(value, current))) {
+			if (!current_value.equals(value)) {
 				from_child = true;
-				set(inner_current_value, mutation ? current : value);
+				set(inner_current_value, value);
 				get(current_value); // force a synchronisation immediately
 			}