allow spring/tweened values to be initially undefined - closes #3761

src/runtime/motion/spring.ts

@@ -65,7 +65,7 @@ interface Spring<T> extends Readable<T>{
 	stiffness: number;
 }
 
-export function spring<T=any>(value: T, opts: SpringOpts = {}): Spring<T> {
+export function spring<T=any>(value?: T, opts: SpringOpts = {}): Spring<T> {
 	const store = writable(value);
 	const { stiffness = 0.15, damping = 0.8, precision = 0.01 } = opts;
 
@@ -84,7 +84,7 @@ export function spring<T=any>(value: T, opts: SpringOpts = {}): Spring<T> {
 		target_value = new_value;
 		const token = current_token = {};
 
-		if (opts.hard || (spring.stiffness >= 1 && spring.damping >= 1)) {
+		if (value == null || opts.hard || (spring.stiffness >= 1 && spring.damping >= 1)) {
 			cancel_task = true; // cancel any running animation
 			last_time = now();
 			last_value = value;

src/runtime/motion/tweened.ts

@@ -69,13 +69,18 @@ interface Tweened<T> extends Readable<T> {
 	update(updater: Updater<T>, opts: Options<T>): Promise<void>;
 }
 
-export function tweened<T>(value: T, defaults: Options<T> = {}): Tweened<T> {
+export function tweened<T>(value?: T, defaults: Options<T> = {}): Tweened<T> {
 	const store = writable(value);
 
 	let task: Task;
 	let target_value = value;
 
 	function set(new_value: T, opts: Options<T>) {
+		if (value == null) {
+			store.set(value = new_value);
+			return Promise.resolve();
+		}
+
 		target_value = new_value;
 
 		let previous_task = task;

test/motion/index.js

@@ -0,0 +1,23 @@
+import * as assert from 'assert';
+import { get } from '../../store';
+import { spring, tweened } from '../../motion';
+
+describe('motion', () => {
+	describe('spring', () => {
+		it('handles initially undefined values', () => {
+			const size = spring();
+
+			size.set(100);
+			assert.equal(get(size), 100);
+		});
+	});
+
+	describe('tweened', () => {
+		it('handles initially undefined values', () => {
+			const size = tweened();
+
+			size.set(100);
+			assert.equal(get(size), 100);
+		});
+	});
+});