fix: make Tween with duration 0 set current to target immediately (#14937)

* fix: Make Tween duration 0 set current to target immediately

* Run prettier on test file

* tweak

---------

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

.changeset/cold-cups-act.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: Make Tween duration 0 set current to target immediately

packages/svelte/src/motion/tweened.js

@@ -230,9 +230,6 @@ export class Tween {
 	set(value, options) {
 		set(this.#target, value);
 
-		let previous_task = this.#task;
-
-		let started = false;
 		let {
 			delay = 0,
 			duration = 400,
@@ -240,10 +237,18 @@ export class Tween {
 			interpolate = get_interpolator
 		} = { ...this.#defaults, ...options };
 
+		if (duration === 0) {
+			this.#task?.abort();
+			set(this.#current, value);
+			return Promise.resolve();
+		}
+
 		const start = raf.now() + delay;
 
 		/** @type {(t: number) => T} */
 		let fn;
+		let started = false;
+		let previous_task = this.#task;
 
 		this.#task = loop((now) => {
 			if (now < start) {

packages/svelte/tests/motion/test.ts

@@ -2,7 +2,7 @@
 import '../helpers.js'; // for the matchMedia polyfill
 import { describe, it, assert } from 'vitest';
 import { get } from 'svelte/store';
-import { spring, tweened } from 'svelte/motion';
+import { spring, tweened, Tween } from 'svelte/motion';
 
 describe('motion', () => {
 	describe('spring', () => {
@@ -39,4 +39,20 @@ describe('motion', () => {
 			assert.equal(get(size), 20);
 		});
 	});
+
+	describe('Tween', () => {
+		it('sets immediately when duration is 0', () => {
+			const size = new Tween(0);
+
+			size.set(100, { duration: 0 });
+			assert.equal(size.current, 100);
+		});
+	});
+
+	it('updates correctly when initialized with a `null`-ish value', () => {
+		const size = new Tween(undefined as unknown as number, { duration: 0 });
+
+		size.set(10);
+		assert.equal(size.current, 10);
+	});
 });