feat: add Spring and Tween classes (#11519)

* feat: add Spring class

* add some docs, Spring.of static method

* add Tween class

* lint

* preserveMomentum in milliseconds

* deprecate tweened

* changeset

* wrestle with types

* more consolidation

* flesh out the distinction a bit more, deprecate `subscribe`

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/tame-bottles-switch.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `Spring` and `Tween` classes to `svelte/motion`

packages/svelte/src/internal/shared/utils.js

@@ -44,6 +44,27 @@ export function run_all(arr) {
 	}
 }
 
+/**
+ * TODO replace with Promise.withResolvers once supported widely enough
+ * @template T
+ */
+export function deferred() {
+	/** @type {(value: T) => void} */
+	var resolve;
+
+	/** @type {(reason: any) => void} */
+	var reject;
+
+	/** @type {Promise<T>} */
+	var promise = new Promise((res, rej) => {
+		resolve = res;
+		reject = rej;
+	});
+
+	// @ts-expect-error
+	return { promise, resolve, reject };
+}
+
 /**
  * @template V
  * @param {V} value

packages/svelte/src/motion/private.d.ts

@@ -1,9 +1,11 @@
-import { Spring } from './public.js';
-
-export interface TickContext<T> {
+export interface TickContext {
 	inv_mass: number;
 	dt: number;
-	opts: Spring<T>;
+	opts: {
+		stiffness: number;
+		damping: number;
+		precision: number;
+	};
 	settled: boolean;
 }
 
@@ -14,8 +16,22 @@ export interface SpringOpts {
 }
 
 export interface SpringUpdateOpts {
+	/**
+	 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+	 */
 	hard?: any;
+	/**
+	 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+	 */
 	soft?: string | number | boolean;
+	/**
+	 * Only use this for the Spring class; does nothing when set on the spring store
+	 */
+	instant?: boolean;
+	/**
+	 * Only use this for the Spring class; does nothing when set on the spring store
+	 */
+	preserveMomentum?: number;
 }
 
 export type Updater<T> = (target_value: T, value: T) => T;

packages/svelte/src/motion/public.d.ts

@@ -1,17 +1,87 @@
-import { Readable } from '../store/public.js';
-import { SpringUpdateOpts, TweenedOptions, Updater } from './private.js';
+import { Readable, type Unsubscriber } from '../store/public.js';
+import { SpringUpdateOpts, TweenedOptions, Updater, SpringOpts } from './private.js';
+
+// TODO we do declaration merging here in order to not have a breaking change (renaming the Spring interface)
+// this means both the Spring class and the Spring interface are merged into one with some things only
+// existing on one side. In Svelte 6, remove the type definition and move the jsdoc onto the class in spring.js
 
 export interface Spring<T> extends Readable<T> {
-	set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
+	set(new_value: T, opts?: SpringUpdateOpts): Promise<void>;
+	/**
+	 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+	 */
 	update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
+	/**
+	 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+	 */
+	subscribe(fn: (value: T) => void): Unsubscriber;
 	precision: number;
 	damping: number;
 	stiffness: number;
 }
 
+/**
+ * A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to
+ * move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.
+ *
+ * ```svelte
+ * <script>
+ * 	import { Spring } from 'svelte/motion';
+ *
+ * 	const spring = new Spring(0);
+ * </script>
+ *
+ * <input type="range" bind:value={spring.target} />
+ * <input type="range" bind:value={spring.current} disabled />
+ * ```
+ */
+export class Spring<T> {
+	constructor(value: T, options?: SpringOpts);
+
+	/**
+	 * Create a spring whose value is bound to the return value of `fn`. This must be called
+	 * inside an effect root (for example, during component initialisation).
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Spring } from 'svelte/motion';
+	 *
+	 * 	let { number } = $props();
+	 *
+	 * 	const spring = Spring.of(() => number);
+	 * </script>
+	 * ```
+	 */
+	static of<U>(fn: () => U, options?: SpringOpts): Spring<U>;
+
+	/**
+	 * Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.
+	 *
+	 * If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.
+	 *
+	 * If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for
+	 * the specified number of milliseconds. This is useful for things like 'fling' gestures.
+	 */
+	set(value: T, options?: SpringUpdateOpts): Promise<void>;
+
+	damping: number;
+	precision: number;
+	stiffness: number;
+	/**
+	 * The end value of the spring.
+	 * This property only exists on the `Spring` class, not the legacy `spring` store.
+	 */
+	target: T;
+	/**
+	 * The current value of the spring.
+	 * This property only exists on the `Spring` class, not the legacy `spring` store.
+	 */
+	get current(): T;
+}
+
 export interface Tweened<T> extends Readable<T> {
 	set(value: T, opts?: TweenedOptions<T>): Promise<void>;
 	update(updater: Updater<T>, opts?: TweenedOptions<T>): Promise<void>;
 }
 
-export * from './index.js';
+export { spring, tweened, Tween } from './index.js';

packages/svelte/src/motion/spring.js

@@ -1,14 +1,18 @@
 /** @import { Task } from '#client' */
 /** @import { SpringOpts, SpringUpdateOpts, TickContext } from './private.js' */
-/** @import { Spring } from './public.js' */
+/** @import { Spring as SpringStore } from './public.js' */
 import { writable } from '../store/shared/index.js';
 import { loop } from '../internal/client/loop.js';
 import { raf } from '../internal/client/timing.js';
 import { is_date } from './utils.js';
+import { set, source } from '../internal/client/reactivity/sources.js';
+import { render_effect } from '../internal/client/reactivity/effects.js';
+import { get } from '../internal/client/runtime.js';
+import { deferred, noop } from '../internal/shared/utils.js';
 
 /**
  * @template T
- * @param {TickContext<T>} ctx
+ * @param {TickContext} ctx
  * @param {T} last_value
  * @param {T} current_value
  * @param {T} target_value
@@ -53,10 +57,11 @@ function tick_spring(ctx, last_value, current_value, target_value) {
 /**
  * The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it "bounces" like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.
  *
+ * @deprecated Use [`Spring`](https://svelte.dev/docs/svelte/svelte-motion#Spring) instead
  * @template [T=any]
  * @param {T} [value]
  * @param {SpringOpts} [opts]
- * @returns {Spring<T>}
+ * @returns {SpringStore<T>}
  */
 export function spring(value, opts = {}) {
 	const store = writable(value);
@@ -103,7 +108,7 @@ export function spring(value, opts = {}) {
 					return false;
 				}
 				inv_mass = Math.min(inv_mass + inv_mass_recovery_rate, 1);
-				/** @type {TickContext<T>} */
+				/** @type {TickContext} */
 				const ctx = {
 					inv_mass,
 					opts: spring,
@@ -127,7 +132,8 @@ export function spring(value, opts = {}) {
 			});
 		});
 	}
-	/** @type {Spring<T>} */
+	/** @type {SpringStore<T>} */
+	// @ts-expect-error - class-only properties are missing
 	const spring = {
 		set,
 		update: (fn, opts) => set(fn(/** @type {T} */ (target_value), /** @type {T} */ (value)), opts),
@@ -138,3 +144,205 @@ export function spring(value, opts = {}) {
 	};
 	return spring;
 }
+
+/**
+ * A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to
+ * move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.
+ *
+ * ```svelte
+ * <script>
+ * 	import { Spring } from 'svelte/motion';
+ *
+ * 	const spring = new Spring(0);
+ * </script>
+ *
+ * <input type="range" bind:value={spring.target} />
+ * <input type="range" bind:value={spring.current} disabled />
+ * ```
+ * @template T
+ * @since 5.8.0
+ */
+export class Spring {
+	#stiffness = source(0.15);
+	#damping = source(0.8);
+	#precision = source(0.01);
+
+	#current = source(/** @type {T} */ (undefined));
+	#target = source(/** @type {T} */ (undefined));
+
+	#last_value = /** @type {T} */ (undefined);
+	#last_time = 0;
+
+	#inverse_mass = 1;
+	#momentum = 0;
+
+	/** @type {import('../internal/client/types').Task | null} */
+	#task = null;
+
+	/** @type {ReturnType<typeof deferred> | null} */
+	#deferred = null;
+
+	/**
+	 * @param {T} value
+	 * @param {SpringOpts} [options]
+	 */
+	constructor(value, options = {}) {
+		this.#current.v = this.#target.v = value;
+
+		if (typeof options.stiffness === 'number') this.#stiffness.v = clamp(options.stiffness, 0, 1);
+		if (typeof options.damping === 'number') this.#damping.v = clamp(options.damping, 0, 1);
+		if (typeof options.precision === 'number') this.#precision.v = options.precision;
+	}
+
+	/**
+	 * Create a spring whose value is bound to the return value of `fn`. This must be called
+	 * inside an effect root (for example, during component initialisation).
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Spring } from 'svelte/motion';
+	 *
+	 * 	let { number } = $props();
+	 *
+	 * 	const spring = Spring.of(() => number);
+	 * </script>
+	 * ```
+	 * @template U
+	 * @param {() => U} fn
+	 * @param {SpringOpts} [options]
+	 */
+	static of(fn, options) {
+		const spring = new Spring(fn(), options);
+
+		render_effect(() => {
+			spring.set(fn());
+		});
+
+		return spring;
+	}
+
+	/** @param {T} value */
+	#update(value) {
+		set(this.#target, value);
+
+		this.#current.v ??= value;
+		this.#last_value ??= this.#current.v;
+
+		if (!this.#task) {
+			this.#last_time = raf.now();
+
+			var inv_mass_recovery_rate = 1000 / (this.#momentum * 60);
+
+			this.#task ??= loop((now) => {
+				this.#inverse_mass = Math.min(this.#inverse_mass + inv_mass_recovery_rate, 1);
+
+				/** @type {import('./private').TickContext} */
+				const ctx = {
+					inv_mass: this.#inverse_mass,
+					opts: {
+						stiffness: this.#stiffness.v,
+						damping: this.#damping.v,
+						precision: this.#precision.v
+					},
+					settled: true,
+					dt: ((now - this.#last_time) * 60) / 1000
+				};
+
+				var next = tick_spring(ctx, this.#last_value, this.#current.v, this.#target.v);
+				this.#last_value = this.#current.v;
+				this.#last_time = now;
+				set(this.#current, next);
+
+				if (ctx.settled) {
+					this.#task = null;
+				}
+
+				return !ctx.settled;
+			});
+		}
+
+		return this.#task.promise;
+	}
+
+	/**
+	 * Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.
+	 *
+	 * If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.
+	 *
+	 * If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for
+	 * the specified number of milliseconds. This is useful for things like 'fling' gestures.
+	 *
+	 * @param {T} value
+	 * @param {SpringUpdateOpts} [options]
+	 */
+	set(value, options) {
+		this.#deferred?.reject(new Error('Aborted'));
+
+		if (options?.instant || this.#current.v === undefined) {
+			this.#task?.abort();
+			this.#task = null;
+			set(this.#current, set(this.#target, value));
+			return Promise.resolve();
+		}
+
+		if (options?.preserveMomentum) {
+			this.#inverse_mass = 0;
+			this.#momentum = options.preserveMomentum;
+		}
+
+		var d = (this.#deferred = deferred());
+		d.promise.catch(noop);
+
+		this.#update(value).then(() => {
+			if (d !== this.#deferred) return;
+			d.resolve(undefined);
+		});
+
+		return d.promise;
+	}
+
+	get current() {
+		return get(this.#current);
+	}
+
+	get damping() {
+		return get(this.#damping);
+	}
+
+	set damping(v) {
+		set(this.#damping, clamp(v, 0, 1));
+	}
+
+	get precision() {
+		return get(this.#precision);
+	}
+
+	set precision(v) {
+		set(this.#precision, v);
+	}
+
+	get stiffness() {
+		return get(this.#stiffness);
+	}
+
+	set stiffness(v) {
+		set(this.#stiffness, clamp(v, 0, 1));
+	}
+
+	get target() {
+		return get(this.#target);
+	}
+
+	set target(v) {
+		this.set(v);
+	}
+}
+
+/**
+ * @param {number} n
+ * @param {number} min
+ * @param {number} max
+ */
+function clamp(n, min, max) {
+	return Math.max(min, Math.min(max, n));
+}

packages/svelte/src/motion/tweened.js

@@ -6,6 +6,8 @@ import { raf } from '../internal/client/timing.js';
 import { loop } from '../internal/client/loop.js';
 import { linear } from '../easing/index.js';
 import { is_date } from './utils.js';
+import { set, source } from '../internal/client/reactivity/sources.js';
+import { get, render_effect } from 'svelte/internal/client';
 
 /**
  * @template T
@@ -76,6 +78,7 @@ function get_interpolator(a, b) {
 /**
  * A tweened store in Svelte is a special type of store that provides smooth transitions between state values over time.
  *
+ * @deprecated Use [`Tween`](https://svelte.dev/docs/svelte/svelte-motion#Tween) instead
  * @template T
  * @param {T} [value]
  * @param {TweenedOptions<T>} [defaults]
@@ -152,3 +155,137 @@ export function tweened(value, defaults = {}) {
 		subscribe: store.subscribe
 	};
 }
+
+/**
+ * A wrapper for a value that tweens smoothly to its target value. Changes to `tween.target` will cause `tween.current` to
+ * move towards it over time, taking account of the `delay`, `duration` and `easing` options.
+ *
+ * ```svelte
+ * <script>
+ * 	import { Tween } from 'svelte/motion';
+ *
+ * 	const tween = new Tween(0);
+ * </script>
+ *
+ * <input type="range" bind:value={tween.target} />
+ * <input type="range" bind:value={tween.current} disabled />
+ * ```
+ * @template T
+ * @since 5.8.0
+ */
+export class Tween {
+	#current = source(/** @type {T} */ (undefined));
+	#target = source(/** @type {T} */ (undefined));
+
+	/** @type {TweenedOptions<T>} */
+	#defaults;
+
+	/** @type {import('../internal/client/types').Task | null} */
+	#task = null;
+
+	/**
+	 * @param {T} value
+	 * @param {TweenedOptions<T>} options
+	 */
+	constructor(value, options = {}) {
+		this.#current.v = this.#target.v = value;
+		this.#defaults = options;
+	}
+
+	/**
+	 * Create a tween whose value is bound to the return value of `fn`. This must be called
+	 * inside an effect root (for example, during component initialisation).
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Tween } from 'svelte/motion';
+	 *
+	 * 	let { number } = $props();
+	 *
+	 * 	const tween = Tween.of(() => number);
+	 * </script>
+	 * ```
+	 * @template U
+	 * @param {() => U} fn
+	 * @param {TweenedOptions<U>} [options]
+	 */
+	static of(fn, options) {
+		const tween = new Tween(fn(), options);
+
+		render_effect(() => {
+			tween.set(fn());
+		});
+
+		return tween;
+	}
+
+	/**
+	 * Sets `tween.target` to `value` and returns a `Promise` that resolves if and when `tween.current` catches up to it.
+	 *
+	 * If `options` are provided, they will override the tween's defaults.
+	 * @param {T} value
+	 * @param {TweenedOptions<T>} [options]
+	 * @returns
+	 */
+	set(value, options) {
+		set(this.#target, value);
+
+		let previous_value = this.#current.v;
+		let previous_task = this.#task;
+
+		let started = false;
+		let {
+			delay = 0,
+			duration = 400,
+			easing = linear,
+			interpolate = get_interpolator
+		} = { ...this.#defaults, ...options };
+
+		const start = raf.now() + delay;
+
+		/** @type {(t: number) => T} */
+		let fn;
+
+		this.#task = loop((now) => {
+			if (now < start) {
+				return true;
+			}
+
+			if (!started) {
+				started = true;
+
+				fn = interpolate(/** @type {any} */ (previous_value), value);
+
+				if (typeof duration === 'function') {
+					duration = duration(/** @type {any} */ (previous_value), value);
+				}
+
+				previous_task?.abort();
+			}
+
+			const elapsed = now - start;
+
+			if (elapsed > /** @type {number} */ (duration)) {
+				set(this.#current, value);
+				return false;
+			}
+
+			set(this.#current, fn(easing(elapsed / /** @type {number} */ (duration))));
+			return true;
+		});
+
+		return this.#task.promise;
+	}
+
+	get current() {
+		return get(this.#current);
+	}
+
+	get target() {
+		return get(this.#target);
+	}
+
+	set target(v) {
+		this.set(v);
+	}
+}

packages/svelte/types/index.d.ts

@@ -1637,15 +1637,84 @@ declare module 'svelte/legacy' {
 }
 
 declare module 'svelte/motion' {
-	import type { MediaQuery } from 'svelte/reactivity';
+	// TODO we do declaration merging here in order to not have a breaking change (renaming the Spring interface)
+	// this means both the Spring class and the Spring interface are merged into one with some things only
+	// existing on one side. In Svelte 6, remove the type definition and move the jsdoc onto the class in spring.js
+
 	export interface Spring<T> extends Readable<T> {
-		set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
+		set(new_value: T, opts?: SpringUpdateOpts): Promise<void>;
+		/**
+		 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+		 */
 		update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
+		/**
+		 * @deprecated Only exists on the legacy `spring` store, not the `Spring` class
+		 */
+		subscribe(fn: (value: T) => void): Unsubscriber;
 		precision: number;
 		damping: number;
 		stiffness: number;
 	}
 
+	/**
+	 * A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to
+	 * move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Spring } from 'svelte/motion';
+	 *
+	 * 	const spring = new Spring(0);
+	 * </script>
+	 *
+	 * <input type="range" bind:value={spring.target} />
+	 * <input type="range" bind:value={spring.current} disabled />
+	 * ```
+	 */
+	export class Spring<T> {
+		constructor(value: T, options?: SpringOpts);
+
+		/**
+		 * Create a spring whose value is bound to the return value of `fn`. This must be called
+		 * inside an effect root (for example, during component initialisation).
+		 *
+		 * ```svelte
+		 * <script>
+		 * 	import { Spring } from 'svelte/motion';
+		 *
+		 * 	let { number } = $props();
+		 *
+		 * 	const spring = Spring.of(() => number);
+		 * </script>
+		 * ```
+		 */
+		static of<U>(fn: () => U, options?: SpringOpts): Spring<U>;
+
+		/**
+		 * Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.
+		 *
+		 * If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.
+		 *
+		 * If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for
+		 * the specified number of milliseconds. This is useful for things like 'fling' gestures.
+		 */
+		set(value: T, options?: SpringUpdateOpts): Promise<void>;
+
+		damping: number;
+		precision: number;
+		stiffness: number;
+		/**
+		 * The end value of the spring.
+		 * This property only exists on the `Spring` class, not the legacy `spring` store.
+		 */
+		target: T;
+		/**
+		 * The current value of the spring.
+		 * This property only exists on the `Spring` class, not the legacy `spring` store.
+		 */
+		get current(): T;
+	}
+
 	export interface Tweened<T> extends Readable<T> {
 		set(value: T, opts?: TweenedOptions<T>): Promise<void>;
 		update(updater: Updater<T>, opts?: TweenedOptions<T>): Promise<void>;
@@ -1672,8 +1741,22 @@ declare module 'svelte/motion' {
 	}
 
 	interface SpringUpdateOpts {
+		/**
+		 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+		 */
 		hard?: any;
+		/**
+		 * @deprecated Only use this for the spring store; does nothing when set on the Spring class
+		 */
 		soft?: string | number | boolean;
+		/**
+		 * Only use this for the Spring class; does nothing when set on the spring store
+		 */
+		instant?: boolean;
+		/**
+		 * Only use this for the Spring class; does nothing when set on the spring store
+		 */
+		preserveMomentum?: number;
 	}
 
 	type Updater<T> = (target_value: T, value: T) => T;
@@ -1684,40 +1767,64 @@ declare module 'svelte/motion' {
 		easing?: (t: number) => number;
 		interpolate?: (a: T, b: T) => (t: number) => T;
 	}
-	/**
-	 * A [media query](https://svelte.dev/docs/svelte/svelte-reactivity#MediaQuery) that matches if the user [prefers reduced motion](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion).
-	 *
-	 * ```svelte
-	 * <script>
-	 * 	import { prefersReducedMotion } from 'svelte/motion';
-	 * 	import { fly } from 'svelte/transition';
-	 *
-	 * 	let visible = $state(false);
-	 * </script>
-	 *
-	 * <button onclick={() => visible = !visible}>
-	 * 	toggle
-	 * </button>
-	 *
-	 * {#if visible}
-	 * 	<p transition:fly={{ y: prefersReducedMotion.current ? 0 : 200 }}>
-	 * 		flies in, unless the user prefers reduced motion
-	 * 	</p>
-	 * {/if}
-	 * ```
-	 * @since 5.7.0
-	 */
-	export const prefersReducedMotion: MediaQuery;
 	/**
 	 * The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it "bounces" like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.
 	 *
+	 * @deprecated Use [`Spring`](https://svelte.dev/docs/svelte/svelte-motion#Spring) instead
 	 * */
 	export function spring<T = any>(value?: T | undefined, opts?: SpringOpts | undefined): Spring<T>;
 	/**
 	 * A tweened store in Svelte is a special type of store that provides smooth transitions between state values over time.
 	 *
+	 * @deprecated Use [`Tween`](https://svelte.dev/docs/svelte/svelte-motion#Tween) instead
 	 * */
 	export function tweened<T>(value?: T | undefined, defaults?: TweenedOptions<T> | undefined): Tweened<T>;
+	/**
+	 * A wrapper for a value that tweens smoothly to its target value. Changes to `tween.target` will cause `tween.current` to
+	 * move towards it over time, taking account of the `delay`, `duration` and `easing` options.
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { Tween } from 'svelte/motion';
+	 *
+	 * 	const tween = new Tween(0);
+	 * </script>
+	 *
+	 * <input type="range" bind:value={tween.target} />
+	 * <input type="range" bind:value={tween.current} disabled />
+	 * ```
+	 * @since 5.8.0
+	 */
+	export class Tween<T> {
+		/**
+		 * Create a tween whose value is bound to the return value of `fn`. This must be called
+		 * inside an effect root (for example, during component initialisation).
+		 *
+		 * ```svelte
+		 * <script>
+		 * 	import { Tween } from 'svelte/motion';
+		 *
+		 * 	let { number } = $props();
+		 *
+		 * 	const tween = Tween.of(() => number);
+		 * </script>
+		 * ```
+		 * 
+		 */
+		static of<U>(fn: () => U, options?: TweenedOptions<U> | undefined): Tween<U>;
+		
+		constructor(value: T, options?: TweenedOptions<T>);
+		/**
+		 * Sets `tween.target` to `value` and returns a `Promise` that resolves if and when `tween.current` catches up to it.
+		 *
+		 * If `options` are provided, they will override the tween's defaults.
+		 * */
+		set(value: T, options?: TweenedOptions<T> | undefined): Promise<void>;
+		get current(): T;
+		set target(v: T);
+		get target(): T;
+		#private;
+	}
 
 	export {};
 }