fix: replace `undefined` with `void 0` to avoid edge case (#15511)

* replace 'undefined' with 'void 0'

* lint

* YALF

* reuse expression

* oops

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/curvy-countries-flow.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: replace `undefined` with `void 0` to avoid edge case

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

@@ -116,8 +116,7 @@ export function VariableDeclaration(node, context) {
 			}
 
 			const args = /** @type {CallExpression} */ (init).arguments;
-			const value =
-				args.length === 0 ? b.id('undefined') : /** @type {Expression} */ (context.visit(args[0]));
+			const value = args.length > 0 ? /** @type {Expression} */ (context.visit(args[0])) : b.void0;
 
 			if (rune === '$state' || rune === '$state.raw') {
 				/**

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

@@ -13,11 +13,11 @@ export function CallExpression(node, context) {
 	const rune = get_rune(node, context.state.scope);
 
 	if (rune === '$host') {
-		return b.id('undefined');
+		return b.void0;
 	}
 
 	if (rune === '$effect.tracking') {
-		return b.literal(false);
+		return b.false;
 	}
 
 	if (rune === '$effect.root') {

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

@@ -45,7 +45,7 @@ export function VariableDeclaration(node, context) {
 						) {
 							const right = node.right.arguments.length
 								? /** @type {Expression} */ (context.visit(node.right.arguments[0]))
-								: b.id('undefined');
+								: b.void0;
 							return b.assignment_pattern(node.left, right);
 						}
 					}
@@ -75,8 +75,7 @@ export function VariableDeclaration(node, context) {
 			}
 
 			const args = /** @type {CallExpression} */ (init).arguments;
-			const value =
-				args.length === 0 ? b.id('undefined') : /** @type {Expression} */ (context.visit(args[0]));
+			const value = args.length > 0 ? /** @type {Expression} */ (context.visit(args[0])) : b.void0;
 
 			if (rune === '$derived.by') {
 				declarations.push(

packages/svelte/src/compiler/utils/builders.js

@@ -154,6 +154,8 @@ export function unary(operator, argument) {
 	return { type: 'UnaryExpression', argument, operator, prefix: true };
 }
 
+export const void0 = unary('void', literal(0));
+
 /**
  * @param {ESTree.Expression} test
  * @param {ESTree.Expression} consequent