fix: increment and decrement edge case (#11506)

* fix: increment and decrement edge case

* fix/simplify test

* simplify

* Apply suggestions from code review

* golf

---------

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

.changeset/chilled-seas-jog.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: coerce incremented/decremented sources

packages/svelte/src/internal/client/runtime.js

@@ -1011,7 +1011,7 @@ function get_parent_context(component_context) {
  * @returns {number}
  */
 export function update(signal, d = 1) {
-	const value = get(signal);
+	var value = +get(signal);
 	set(signal, value + d);
 	return value;
 }
@@ -1022,9 +1022,7 @@ export function update(signal, d = 1) {
  * @returns {number}
  */
 export function update_pre(signal, d = 1) {
-	const value = get(signal) + d;
-	set(signal, value);
-	return value;
+	return set(signal, +get(signal) + d);
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/increment-and-decrement-strings/_config.js

@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<button>update</button>
+		<p>0, 0, 0, 0</p>
+	`,
+
+	test({ target, assert, logs }) {
+		const btn = target.querySelector('button');
+		flushSync(() => btn?.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>update</button>
+				<p>1, -1, 1, -1</p>
+			`
+		);
+
+		assert.deepEqual(logs, [0, 0, 1, -1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/increment-and-decrement-strings/main.svelte

@@ -0,0 +1,20 @@
+<script>
+	let a = $state("0");
+	let b = $state("0");
+	let c = $state("0");
+	let d = $state("0");
+
+	function update() {
+		// @ts-expect-error
+		console.log(a++);
+		// @ts-expect-error
+		console.log(b--);
+		// @ts-expect-error
+		console.log(++c);
+		// @ts-expect-error
+		console.log(--d);
+	}
+</script>
+
+<button onclick={update}>update</button>
+<p>{a}, {b}, {c}, {d}</p>