ScrollMagic

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The javascript library for magical scroll interactions.

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# ScrollMagic 3

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### The lightweight library for magical scroll interactions

> **Looking for ScrollMagic v2?** The legacy version is on the [`v2-stable`](https://github.com/janpaepke/ScrollMagic/tree/v2-stable) branch.

ScrollMagic tells you where an element is relative to the viewport as the user scrolls — and fires events when that changes.

It's a convenience wrapper around [IntersectionObserver](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver) and [ResizeObserver](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver) that handles the performance pitfalls and counter-intuitive edge cases for you.

[![Donate](https://scrollmagic.io/assets/img/btn_donate.svg 'Shut up and take my money!')](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=8BJC8B58XHKLL 'Shut up and take my money!')

### Not an animation library – unless you want it to be

By itself, ScrollMagic doesn't animate anything. It provides precise scroll-position data and events — what you do with them is up to you. If you're looking for a ready-made scroll animation solution, check out [GSAP ScrollTrigger](https://gsap.com/docs/v3/Plugins/ScrollTrigger/), [Motion](https://motion.dev/docs/scroll), or [anime.js](https://animejs.com/).

For pure CSS-driven scroll animations, see native [scroll-driven animations](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_scroll-driven_animations) (not yet supported in all browsers). ScrollMagic complements them by providing cross-browser support, event callbacks, progress values, and state management that the native API doesn't cover.

ScrollMagic is a general-purpose, framework-agnostic, zero-dependency foundation for scroll-driven UX — what you do with it is entirely up to you: class toggles, animations, lazy loading, parallax, scroll-linked video, behavioural tracking, or anything else.

### Why ScrollMagic?

- Tiny footprint, zero dependencies
- Free to use ([open source](LICENSE.md))
- Optimized for performance (shared observers, batched rAF, single-frame updates)
- Built for modern browsers, mobile compatible
- Native TypeScript support
- SSR safe
- Works with any scroll container (window or custom element)
- Horizontal and vertical scrolling
- Plugin system for extensibility
- Framework agnostic — works with React, Vue, vanilla JS, anything

## Installation

```sh
npm install scrollmagic@next
```

## Quick Start

```js
import ScrollMagic from 'scrollmagic';

new ScrollMagic({ element: '#my-element' })
	.on('enter', () => console.log('visible!'))
	.on('leave', () => console.log('gone!'))
	.on('progress', e => console.log(`${(e.target.progress * 100).toFixed(0)}%`));
```

## How It Works

ScrollMagic uses two sets of bounds to define the active range:

- **Container bounds** — a zone on the scroll container, defined by `containerStart` and `containerEnd`
- **Element bounds** — a zone on the tracked element, defined by `elementStart` and `elementEnd`

Progress goes from `0` to `1` as the element bounds pass through the container bounds. Events fire on enter, leave, and progress change.

### Contain and Intersect

The two most common configurations are **contain** and **intersect**. They differ in where the container bounds are positioned:

#### Contain (default when `element` is `null`)

The container bounds match the viewport edges — `containerStart` and `containerEnd` are both at `'here'` (`0%`). Progress goes from 0 to 1 while one fully **contains** the other: either the element is fully visible inside the viewport, or the element fully covers the viewport.

Typical uses: scroll progress bars, parallax, scroll-linked video, scroll-driven storytelling.

#### Intersect (default when `element` is set)

The container bounds span the full viewport — `containerStart` and `containerEnd` are at `'opposite'` edges (`100%`). Progress goes from 0 to 1 while the element **intersects** with the viewport: starting when its leading edge enters and ending when its trailing edge leaves.

Typical uses: enter/leave animations, lazy loading, class toggles, visibility tracking.

#### Not just defaults

While _contain_ and _intersect_ are the inferred defaults, you can also configure them explicitly — for example setting `containerStart: 0, containerEnd: 0` on an instance that has an element to get contain behaviour, or mixing container and element insets for custom tracking zones. The two configurations are **useful mental models, not rigid modes**.

#### Native scroll-driven animation ranges

If you're familiar with [CSS scroll-driven animations](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_scroll-driven_animations), here's how the native `view()` timeline ranges map to ScrollMagic configurations:

| Native range | ScrollMagic equivalent |
| ------------ | ---------------------- |
| `cover`      | _intersect_ default — `containerStart: 'opposite', containerEnd: 'opposite'` |
| `contain`    | _contain_ default — `containerStart: 0, containerEnd: 0` |
| `entry`      | `containerStart: 'opposite', containerEnd: 0` — container zone collapses to the trailing edge |
| `exit`       | `containerStart: 0, containerEnd: 'opposite'` — container zone collapses to the leading edge |

The native `entry-crossing` and `exit-crossing` ranges are equivalent to `entry` and `exit` above — the distinction only applies when subdividing a single native timeline, not when defining standalone tracking ranges.

## Options

All options are optional. They can be passed to the constructor and updated at any time via setters or `.modify()`.

| Option           | Type                                   | Default                    | Description                                           |
| ---------------- | -------------------------------------- | -------------------------- | ----------------------------------------------------- |
| `element`        | `Element \| string \| null`            | first child of `container` | The tracked element (or CSS selector). Selectors match only the first element — create one instance per element to track multiple. |
| `elementStart`   | `number \| string \| function`         | `0`                        | Start **inset** on the element.                       |
| `elementEnd`     | `number \| string \| function`         | `0`                        | End **inset** on the element.                         |
| `container`      | `Window \| Element \| string \| null`  | `window`                   | The scroll container (or CSS selector). Selectors use the first match. |
| `containerStart` | `number \| string \| function \| null` | inferred (see below)       | Start **inset** on the scroll container.              |
| `containerEnd`   | `number \| string \| function \| null` | inferred (see below)       | End **inset** on the scroll container.                |
| `vertical`       | `boolean`                              | `true`                     | Scroll axis. `true` = vertical, `false` = horizontal. |

**Inset values** work like CSS `top`/`bottom`: positive values offset inward from the respective edge in the tracked direction. Accepted value types:

- **Numbers** — pixel values (e.g. `50`)
- **Strings** — percentage or pixel strings (e.g. `'50%'`, `'20px'`), relative to the parent size (scroll container for container options, element for element options)
- **Named positions** — `'here'` (0%), `'center'` (50%), `'opposite'` (100%)
- **Functions** — `(size) => number` for dynamic computation

**`null` means infer:** For `element`, `container`, `containerStart`, or `containerEnd`, setting it to `null` resets them to their inferred default.

For `containerStart`/`containerEnd` the inferred values depend on `element`:

- **`element` is `null`** → defaults to [**contain**](#contain-default-when-element-is-null): the element is inferred as the first child of the container (for `window` this is `document.body`), container offsets are `'here'` (0%), mapping progress to overall scroll position.
- **`element` is not `null`** → defaults to [**intersect**](#intersect-default-when-element-is-set): container offsets are `'opposite'` (100%), tracking the element as it scrolls through the full viewport.

## Events

Subscribe with `.on()`, `.off()`, or `.subscribe()` (returns an unsubscribe function). Pass `{ once: true }` to auto-remove the listener after its first invocation. Calling `.off()` or the unsubscribe function after the listener has already been removed (e.g. after a `once` listener fires) is a safe no-op.

| Event      | When                                                     |
| ---------- | -------------------------------------------------------- |
| `enter`    | Element enters the active zone (progress leaves 0 or 1)  |
| `leave`    | Element leaves the active zone (progress reaches 0 or 1) |
| `progress` | Progress value changes while in the active zone          |

Every event provides:

## Examples

```js
// Intersect (default): active while any part of the element
// is visible in the viewport
new ScrollMagic({
	element: '#a',
});

// Intersect with narrowed container zone:
// active while the element passes through the center line
new ScrollMagic({
	element: '#b',
	containerStart: 'center',
	containerEnd: 'center',
});

// Same as above, but with element offsets:
// starts 50px before the element, ends 100px after it
new ScrollMagic({
	element: '#c',
	containerStart: 'center',
	containerEnd: 'center',
	elementStart: -50,
	elementEnd: -100,
});

// Fixed scroll distance of 150px, regardless of element height.
// elementEnd receives the element's size and offsets from
// the bottom — (size - 150) leaves only 150px of track.
new ScrollMagic({
	element: '#d',
	containerStart: 'center',
	containerEnd: 'center',
	elementEnd: size => size - 150,
});

// Contain: active only while the element is fully visible
// (element insets pushed to opposite edges = full element height)
new ScrollMagic({
	element: '#e',
	elementStart: 'opposite', // same as '100%'
	elementEnd: 'opposite', // same as '100%'
});

// Contain (default when no element): track overall scroll progress
new ScrollMagic();
```

## API

```ts
const sm = new ScrollMagic(options);

// Event listeners
sm.on(type, callback); // add listener, returns instance (chainable)
sm.on(type, callback, { once: true }); // listener auto-removes after first invocation
sm.off(type, callback); // remove listener, returns instance (chainable)
sm.subscribe(type, callback); // add listener, returns unsubscribe function
sm.subscribe(type, callback, { once: true }); // both auto-removes and returns unsubscribe

// Modify options after creation
sm.modify({ containerStart: 'center' });

// All options can also be directly read and written
const elem = sm.element; // get the tracked element
sm.containerStart = 'center'; // set individual options

// Read-only getters
sm.progress; // 0–1, how far through the active zone
sm.activeRange; // { start, end } container scroll positions where tracking is active
sm.scrollVelocity; // px/s along tracked axis, 0 when idle
sm.resolvedBounds; // { element, container } cached layout bounds

// Refresh — recalculate bounds after external layout changes
sm.refresh();

// Pause / resume tracking without destroying
sm.disable(); // disconnects all observers, freezes progress
sm.enable(); // reconnects observers, recalculates from current state
sm.disabled; // read-only, true when disabled or destroyed

// Lifecycle
sm.destroy();

// Static
ScrollMagic.defaultOptions({ vertical: false }); // get/set defaults for new instances
ScrollMagic.refreshAll(); // refresh every active instance
ScrollMagic.destroyAll(); // destroy every active instance
```

## When to use `refresh()`

ScrollMagic automatically tracks element size changes (via `ResizeObserver`) and scroll position changes. But some layout changes are invisible to these observers — they change an element's **position** without changing its **size** or triggering a scroll event.

Call `refresh()` (or `ScrollMagic.refreshAll()`) after:

- **CSS position/margin/padding changes** — `element.style.marginTop = '20px'`
- **CSS class toggles that affect layout** — `element.classList.add('expanded')`
- **DOM structure changes** — siblings added/removed above the element, shifting its position
- **Images loading without explicit dimensions** — an `<img>` above the tracked element loads and expands, pushing it down
- **Font loading** — `document.fonts.ready.then(() => ScrollMagic.refreshAll())`
- **Route changes in SPAs** — content swap changes scroll height
- **Dynamic content loading** — CMS-injected content, third-party widgets

```js
// After changing a style that affects position
element.style.marginTop = '100px';
sm.refresh();

// After fonts finish loading (affects text reflow)
document.fonts.ready.then(() => ScrollMagic.refreshAll());

// After a framework re-render that changes layout
onRouteChange(() => ScrollMagic.refreshAll());
```

Note that `refresh()` is only needed if you want bounds to update **before the next scroll event**. If the user keeps scrolling, element positions are re-read on every scroll frame anyway. `refresh()` matters when layout changes while tracking is active and the scroll position stays the same — e.g. toggling a class or injecting content without any scrolling.

`refresh()` is asynchronous — it schedules recalculation for the next animation frame and returns immediately. Multiple `refresh()` calls within the same frame are batched automatically.

## Plugins

ScrollMagic has a plugin system for extending instance behaviour.

```ts
sm.addPlugin(myPlugin);
sm.removePlugin(myPlugin);
```

See [PLUGINS.md](PLUGINS.md) for the full plugin authoring guide.

## Browser Support

Chrome 73+, Firefox 69+, Safari 13.1+, Edge 79+ (aligned to `ResizeObserver` support).

## License

MIT — [Jan Paepke](https://janpaepke.de)

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