@magic-spells/physics-engine

Spring physics engine for smooth, natural animations. Drives a numeric value from start to end using a spring-damper model, emitting position and progress on every frame.

🔍 Live Demo - See it in action!

Uses

• @magic-spells/event-emitter — the on/off/emit/removeAllListeners base class this engine extends. The only dependency, installed automatically with this package.

The ESM build lists it as a dependency rather than bundling it, so a project that also pulls in other @magic-spells packages resolves a single shared copy instead of one per package. The UMD build bundles it, since a script tag has no npm to dedupe with — that inlined copy is the whole 0.09 kB difference between the two size numbers below.

Size & scope

1.35 kB gzipped (ESM core — event-emitter installs alongside via npm) · 1.44 kB gzipped (UMD core, script-tag ready, self-contained). Ships TypeScript definitions.

This is a 1D interpolator, not a rigid-body or collision engine. It gives you one numeric value and a progress ratio per frame, frame-rate independent, and leaves it to you to map those onto whatever you're animating. You do not get collisions, multi-body simulation, or a scrubbing playhead.

Install

npm install @magic-spells/physics-engine

import PhysicsEngine from '@magic-spells/physics-engine';

<!-- CDN (UMD) — exposes window.PhysicsEngine, self-contained -->
<script src="https://unpkg.com/@magic-spells/physics-engine/dist/physics-engine.min.js"></script>

Usage

const spring = new PhysicsEngine({ attraction: 0.026, friction: 0.28 });

spring.on('change', ({ position, progress }) => {
  element.style.transform = `translateX(${position}px)`;
});

await spring.animateTo(0, 400);

Chained animations with initial velocity:

const spring = new PhysicsEngine({ attraction: 0.02, friction: 0.14 });

spring.on('change', ({ position, progress }) => {
  element.style.opacity = progress;
  element.style.transform = `translateY(${position}px)`;
});

// Animate forward, then back with carry-over velocity
await spring.animateTo(0, 300, 100);
await spring.animateTo(300, 0, -50);

API reference

new PhysicsEngine({ attraction?, friction? })

Create a spring engine.

• attraction number — Spring stiffness. Default 0.026. Must be in range (0, 1) exclusive.
• friction number — Velocity damping. Default 0.28. Must be in range (0, 1) exclusive.

Throws if either value is outside the valid range.

animateTo(startValue, endValue, velocity?) -> Promise

Animate from startValue to endValue. Returns a Promise that resolves when the spring settles or stop() is called.

• startValue number — Starting position.
• endValue number — Target position.
• velocity number — Initial velocity. Default 0.

If called while already animating, the previous animation is silently stopped and its Promise resolves.

stop()

Halts the current animation. Emits a stop event and resolves the pending Promise.

getVelocity()

Returns the current velocity, in units per 16.66ms frame — the same units animateTo() accepts, so it can be handed straight back in to retarget an animation without losing momentum.

let current = 0;
spring.on('change', ({ position }) => { current = position; });

// Redirect mid-flight without losing momentum
spring.animateTo(current, newTarget, spring.getVelocity());

setAttraction(n)

Update attraction while running or idle. Throws if n is not in (0, 1).

setFriction(n)

Update friction while running or idle. Throws if n is not in (0, 1).

isAnimating

Boolean property. true while an animation is in progress.

Events

Event	Payload	When
change	{ position, progress }	Every animation frame
complete	{ position, progress }	Spring has settled at the target
stop	{ position }	stop() was called

Register and remove listeners with on, off, and removeAllListeners:

function onChange({ position, progress }) { /* ... */ }

spring.on('change', onChange);
spring.off('change', onChange);
spring.removeAllListeners('change'); // remove all for one event
spring.removeAllListeners();         // remove all listeners

All three methods return the instance for chaining.

How it works

The engine solves the damped harmonic oscillator analytically and evaluates it at the elapsed time on every frame. attraction is the spring constant and friction is the per-frame velocity decay; together they give a natural frequency of √attraction and a damping ratio of −ln(1 − friction) / (2√attraction), so all three regimes — underdamped, critically damped and overdamped — are covered. The animation settles when both position and velocity are within 0.01 of the target.

Because position is a function of elapsed time rather than an accumulation of per-frame steps, the trajectory does not depend on how the frames land. A 30Hz display, a 144Hz display and a page that drops a frame all follow the same path; refresh rate only decides how often that path is sampled.

Time is measured in 16.66ms units, which is what makes attraction, friction and the velocity animateTo() accepts frame-relative quantities rather than per-second ones.

Changelog

Breaking in 2.0.0. The package is now ESM-only — the require condition has been removed, so CommonJS consumers must either move to import or load the UMD build directly. EventEmitter is no longer vendored into the source; it is now a dependency on @magic-spells/event-emitter, which npm installs automatically. Also in this release: the analytic solver described below, TypeScript definitions, and a Vite 8 two-pass build.

Frame-rate independence (2.0.0). Earlier versions integrated the spring one frame at a time and scaled each step by the frame delta. That made the motion depend on refresh rate — overshoot measured about 3% lower at 30Hz than at 144Hz — and a single dropped frame stretched the animation, because deltas were clamped at 64ms and the excess was discarded. Curves are very slightly springier now: the old integrator consistently undershot the true solution.

License

MIT

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Made by Cory Schulz