The only required field is the target field. This is the field to which the plugin is applied to.
If you use the object initialization only with the target field, the outcome is equivalent to the element initialization:

// Both initializations have the same outcome

OverlayScrollbars(document.querySelector('#myElement'), {});
OverlayScrollbars({ target: document.querySelector('#myElement') }, {});

In the initialization object you can specify how the library is handling generated elements. For example you can appoint an existing element as the viewport element. Like this the library won't generate it but take the specified element instead:

OverlayScrollbars({ 
 target: document.querySelector('#target'),
 elements: {
   viewport: document.querySelector('#viewport'),
 },
}, {});

This is very useful if you have a fixed DOM structure and don't want OverlayScrollbars to generate its own elements. Those cases arise very often when you want an other library to work together with OverlayScrollbars.

You can also decide to which element the scrollbars should be applied to:

OverlayScrollbars({ 
 target: document.querySelector('#target'),
 scrollbars: {
   slot: document.querySelector('#target').parentElement,
 },
}, {});

And last but not least you can decide when the initialization should be canceled:

OverlayScrollbars({ 
 target: document.querySelector('#target'),
 cancel: {
   nativeScrollbarsOverlaid: true,
   body: null,
 }
}, {});

In the above example the initialization is canceled when the native scrollbars are overlaid or when your target is a body element and the plugin determined that a initialization to the body element would affect native functionality like window.scrollTo.

The default options are:

const defaultOptions = {
  paddingAbsolute: false,
  showNativeOverlaidScrollbars: false,
  update: {
    elementEvents: [['img', 'load']],
    debounce: [0, 33],
    attributes: null,
    ignoreMutation: null,
  },
  overflow: {
    x: 'scroll',
    y: 'scroll',
  },
  scrollbars: {
    theme: 'os-theme-dark',
    visibility: 'auto',
    autoHide: 'never',
    autoHideDelay: 1300,
    autoHideSuspend: false,
    dragScroll: true,
    clickScroll: false,
    pointers: ['mouse', 'touch', 'pen'],
  },
};

paddingAbsolute

Indicates whether the padding for the content shall be absolute.

showNativeOverlaidScrollbars

Indicates whether the native overlaid scrollbars shall be visible.

update.elementEvents

An array of tuples. The first value in the tuple is an selector and the second value are event names. The plugin will update itself if any of the elements with the specified selector will emit any specified event. The default value can be interpreted as "The plugin will update itself if any img element emits an load event."

update.debounce

> Note: If 0 is used for the timeout, requestAnimationFrame instead of setTimeout is used for the debounce.

Debounces the MutationObserver which tracks changes to the content. If a tuple is passed, the first value is the timeout and second is the max wait. If only a number is passed you specify only the timeout and there is no max wait. With null there is no debounce. Usefull to fine-tune performance.

update.attributes

> Note: There is a base array of attributes that the MutationObserver always observes, even if this option is null.

An array of additional attributes that the MutationObserver should observe for the content.

update.ignoreMutation

A function which receives a MutationRecord as an argument. If the function returns a truthy value the mutation will be ignored and the plugin won't update. Usefull to fine-tune performance.

overflow.x

> Note: Valid values are: 'hidden', 'scroll', 'visible', 'visible-hidden' and 'visible-scroll'.

The overflow behavior for the horizontal (x) axis.

overflow.y

> Note: Valid values are: 'hidden', 'scroll', 'visible', 'visible-hidden' and 'visible-scroll'.

The overflow behavior for the vertical (y) axis.

scrollbars.theme

Applies the specified theme (classname) to the scrollbars.

scrollbars.visibility

> Note: Valid values are: 'visible', 'hidden', and 'auto'.

The visibility of a scrollbar if its scroll axis is able to have a scrollable overflow. (Scrollable overflow for an axis is only possible with the overflow behavior 'scroll' or 'visible-scroll').

scrollbars.autoHide

> Note: Valid values are: 'never', 'scroll', 'leave' and 'move'.

The possibility to hide visible scrollbars automatically after a certain user action.

scrollbars.autoHideDelay

The delay in milliseconds before the scrollbars are hidden automatically.

scrollbars.autoHideSuspend

Suspend the autoHide functionality until the first scroll interaction was performed.
The default value for this option is false for backwards compatibility reasons but is recommended to be true for better accessibility.

scrollbars.dragScroll

Indicates whether you can drag the scrollbar handles for scrolling.

scrollbars.clickScroll

> Note: This options requires the ClickScrollPlugin to work.

Indicates whether you can click on the scrollbar track for scrolling.

scrollbars.pointers

The PointerTypes the plugin should react to.

TypeScript

// The options of a OverlayScrollbars instance.
export type Options = {
  // Whether the padding shall be absolute.
  paddingAbsolute: boolean;
  // Whether to show the native scrollbars. Has only an effect it the native scrollbars are overlaid.
  showNativeOverlaidScrollbars: boolean;
  // Customizes the automatic update behavior.
  update: {
    /**
     * The given Event(s) from the elements with the given selector(s) will trigger an update.
     * Useful for everything the MutationObserver and ResizeObserver can't detect
     * e.g.: and Images `load` event or the `transitionend` / `animationend` events.
     */
    elementEvents: Array<[elementSelector: string, eventNames: string]> | null;
    /**
     * The debounce which is used to detect content changes.
     * If a tuple is provided you can customize the `timeout` and the `maxWait` in milliseconds.
     * If a single number customizes only the `timeout`.
     *
     * If the `timeout` is `0`, a debounce still exists. (its executed via `requestAnimationFrame`).
     */
    debounce: [timeout: number, maxWait: number] | number | null;
    /**
     * HTML attributes which will trigger an update if they're changed.
     * Basic attributes like `id`, `class`, `style` etc. are always observed and doesn't have to be added explicitly.
     */
    attributes: string[] | null;
    // A function which makes it possible to ignore a content mutation or null if nothing shall be ignored.
    ignoreMutation: ((mutation: MutationRecord) => any) | null;
  };
  // Customizes the overflow behavior per axis.
  overflow: {
    // The overflow behavior of the horizontal (x) axis.
    x: OverflowBehavior;
    // The overflow behavior of the vertical (y) axis.
    y: OverflowBehavior;
  };
  // Customizes appearance of the scrollbars.
  scrollbars: {
    // The scrollbars theme. The theme value will be added as `class` to all `scrollbar` elements of the instance.
    theme: string | null;
    // The scrollbars visibility behavior.
    visibility: ScrollbarsVisibilityBehavior;
    // The scrollbars auto hide behavior.
    autoHide: ScrollbarsAutoHideBehavior;
    // The scrollbars auto hide delay in milliseconds.
    autoHideDelay: number;
    // Whether the scrollbars auto hide behavior is suspended until a scroll happened.
    autoHideSuspend: boolean;
    // Whether its possible to drag the handle of a scrollbar to scroll the viewport.
    dragScroll: boolean;
    // Whether its possible to click the track of a scrollbar to scroll the viewport.
    clickScroll: boolean;
    // An array of pointer types which shall be supported.
    pointers: string[] | null;
  };
};

> Note: Every event receives the instance from which it was dispatched as the first argument. Always.

initialized

Is dispatched after all generated elements, observers and events were appended to the DOM.

updated

> Note: If an update was triggered but nothing changed, the event won't be dispatched.

Is dispatched after the instace was updated.

destroyed

Is dispatched after all generated elements, observers and events were removed from the DOM.

scroll

Is dispatched by scrolling the viewport.

TypeScript

// A mapping between event names and their listener arguments.
export type EventListenerArgs = {
  // Dispatched after all elements are initialized and appended.
  initialized: [instance: OverlayScrollbars];
  // Dispatched after an update.
  updated: [instance: OverlayScrollbars, onUpdatedArgs: OnUpdatedEventListenerArgs];
  // Dispatched after all elements, observers and events are destroyed.
  destroyed: [instance: OverlayScrollbars, canceled: boolean];
  // Dispatched on scroll.
  scroll: [instance: OverlayScrollbars, event: Event];
};

export interface OnUpdatedEventListenerArgs {
  // Hints which describe what changed in the DOM.
  updateHints: {
    // Whether the size of the host element changed.
    sizeChanged: boolean;
    // Whether the direction of the host element changed.
    directionChanged: boolean;
    // Whether the intrinsic height behavior changed.
    heightIntrinsicChanged: boolean;
    // Whether the overflow edge (clientWidth / clientHeight) of the viewport element changed.
    overflowEdgeChanged: boolean;
    // Whether the overflow amount changed.
    overflowAmountChanged: boolean;
    // Whether the overflow style changed.
    overflowStyleChanged: boolean;
    // Whether an host mutation took place.
    hostMutation: boolean;
    // Whether an content mutation took place.
    contentMutation: boolean;
  };
  // The changed options.
  changedOptions: PartialOptions;
  // Whether the update happened with an force invalidated cache.
  force: boolean;
}

options(): Options

Get the current options of the instance.

options(newOptions, pure?): Options

Sets the current options of the instance.

on(eventListeners, pure?): Function

Adds event listeners to the instance.

on(name, listener): Function

Adds a single event listener to the instance.

on(name, listeners): Function

Adds multiple event listeners to the instance.

off(name, listener): void

Removes a single event listener from the instance.

off(name, listeners): void

Removes multiple event listeners from the instance.

update(force?): boolean

Updates the instance.

state(): State

Gets the instances state.

elements(): Elements

Gets the instances elments.

destroy(): void

Destroys the instance and removes all added elements.

plugin(plugin: object): object | undefined

Gets the instance modules instance of the passed plugin.

TypeScript

// A simplified version of the OverlayScrollbars TypeScript interface.
interface OverlayScrollbars {
  // Get the current options of the instance.
  options(): Options;
  // Sets the current options of the instance.
  options(newOptions: PartialOptions, pure?: boolean): Options;

  // Adds event listeners to the instance.
  on(eventListeners: EventListeners, pure?: boolean): () => void;
  // Adds a single event listener to the instance.
  on(name: N, listener: EventListener): () => void;
  // Adds multiple event listeners to the instance.
  on(name: N, listener: EventListener[]): () => void;

  // Removes a single event listener from the instance.
  off(name: N, listener: EventListener): void;
  // Removes multiple event listeners from the instance.
  off(name: N, listener: EventListener[]): void;

  // Updates the instance.
  update(force?: boolean): boolean;

  // Gets the instances state.
  state(): State;

  // Gets the instances elements.
  elements(): Elements;

  // Destroys the instance and removes all added elements.
  destroy(): void;

  // Gets the instance modules instance of the passed plugin.
  plugin<p>(osPlugin: P): InferInstancePluginModuleInstance</p><p> | undefined;
}

// Describes a OverlayScrollbars instances state.
interface State {
  // Describes the current padding in pixel.
  padding: TRBL;
  // Whether the current padding is absolute.
  paddingAbsolute: boolean;
  // The client width (x) &amp; height (y) of the viewport in pixel.
  overflowEdge: XY;
  // The overflow amount in pixel.
  overflowAmount: XY;
  // The css overflow style of the viewport.
  overflowStyle: XY;
  // Whether the viewport has an overflow.
  hasOverflow: XY;
  // Whether the direction is considered rtl.
  directionRTL: boolean;
  // Whether the instance is considered destroyed.
  destroyed: boolean;
}

// Describes the elements of a OverlayScrollbars instance.
interface Elements {
  // The element the instance was applied to.
  target: HTMLElement;
  // The host element. Its the root of all other elements.
  host: HTMLElement;
  /**
   * The element which is responsible to apply correct paddings.
   * Depending on the Initialization it can be the same as the viewport element.
   */
  padding: HTMLElement;
  // The element which is responsible to do any scrolling.
  viewport: HTMLElement;
  /**
   * The element which is responsible to hold the content.
   * Depending on the Initialization it can be the same as the viewport element.
   */
  content: HTMLElement;
  /**
   * The element through which you can get the current `scrollLeft` or `scrollTop` offset.
   * Depending on the target element it can be the same as the viewport element.
   */
  scrollOffsetElement: HTMLElement;
  /**
   * The element through which you can add `scroll` events.
   * Depending on the target element it can be the same as the viewport element.
   */
  scrollEventElement: HTMLElement | Document;
  // The horizontal scrollbar elements.
  scrollbarHorizontal: CloneableScrollbarElements;
  // The vertical scrollbar elements.
  scrollbarVertical: CloneableScrollbarElements;
}

plugin(plugin): object | undefined

Adds a single plugin.

plugin(plugins): (object | void)[]

Adds multiple plugins.

valid(osInstance): boolean

Checks whether the passed value is a valid and not destroyed overlayscrollbars instance

env(): Environment

Gets the environment.

TypeScript

// The OverlayScrollbars static object.
interface OverlayScrollbarsStatic {
  // Gets the instance of the passed target or `undefined` the target has no instance.
  (target: InitializationTarget): OverlayScrollbars | undefined;
  // Initializes OverlayScrollbars to the passed target with passed options and event listeners.
  (target: InitializationTarget, options: PartialOptions, eventListeners?: EventListeners): OverlayScrollbars;

  // Adds a single plugin.
  plugin(plugin: Plugin): InferStaticPluginModuleInstance
  // Adds multiple plugins.
  plugin(plugins: Plugin[]): InferStaticPluginModuleInstance[];

  // Checks whether the passed value is a valid and not destroyed overlayscrollbars instance.
  valid(osInstance: any): osInstance is OverlayScrollbars;

  // Gets the environment.
  env(): Environment;
}

// Describes the OverlayScrollbars environment.
interface Environment {
  // The native scrollbars size of the browser / system.
  scrollbarsSize: XY;
  // Whether the native scrollbars are overlaid.
  scrollbarsOverlaid: XY;
  // Whether the browser supports native scrollbars hiding.
  scrollbarsHiding: boolean;
  // The rtl scroll behavior of the browser.
  rtlScrollBehavior: { n: boolean; i: boolean };
  // Whether the browser supports the ScrollTimeline API.
  scrollTimeline: boolean;
  // The default Initialization to use if nothing else is specified.
  staticDefaultInitialization: Initialization;
  // The default Options to use if nothing else is specified.
  staticDefaultOptions: Options;

  // Returns the current default Initialization.
  getDefaultInitialization(): Initialization;
  // Returns the current default Options.
  getDefaultOptions(): Options;

  /**
   * Sets a new default Initialization.
   * If the new default Initialization is partially filled, its deeply merged with the current default Initialization.
   * @param newDefaultInitialization The new default Initialization.
   * @returns The current default Initialization.
   */
  setDefaultInitialization(newDefaultInitialization: PartialInitialization): Initialization;
  /**
   * Sets new default Options.
   * If the new default Options are partially filled, they're deeply merged with the current default Options.
   * @param newDefaultOptions The new default Options.
   * @returns The current default options.
   */
  setDefaultOptions(newDefaultOptions: PartialOptions): Options;
}

CSS Custom properties

OverlayScrollbars provides a set of CSS Custom Properties which makes scrollbar styling easy and fast:

.os-scrollbar {
  // The size of the scrollbar
  --os-size: 0;
  // The axis-perpedicular padding of the scrollbar (horizontal: padding-y, vertical: padding-x)
  --os-padding-perpendicular: 0;
  // The axis padding of the scrollbar (horizontal: padding-x, vertical: padding-y)
  --os-padding-axis: 0;
  // The border radius of the scrollbar track
  --os-track-border-radius: 0;
  // The background of the scrollbar track
  --os-track-bg: none;
  // The :hover background of the scrollbar track
  --os-track-bg-hover: none;
  // The :active background of the scrollbar track
  --os-track-bg-active: none;
  // The border of the scrollbar track
  --os-track-border: none;
  // The :hover background of the scrollbar track
  --os-track-border-hover: none;
  // The :active background of the scrollbar track
  --os-track-border-active: none;
  // The border radius of the scrollbar handle
  --os-handle-border-radius: 0;
  // The background of the scrollbar handle
  --os-handle-bg: none;
  // The :hover background of the scrollbar handle
  --os-handle-bg-hover: none;
  // The :active background of the scrollbar handle
  --os-handle-bg-active: none;
  // The border of the scrollbar handle
  --os-handle-border: none;
  // The :hover border of the scrollbar handle
  --os-handle-border-hover: none;
  // The :active border of the scrollbar handle
  --os-handle-border-active: none;
  // The min size of the scrollbar handle
  --os-handle-min-size: 33px;
  // The max size of the scrollbar handle
  --os-handle-max-size: none;
  // The axis-perpedicular size of the scrollbar handle (horizontal: height, vertical: width)
  --os-handle-perpendicular-size: 100%;
  // The :hover axis-perpedicular size of the scrollbar handle (horizontal: height, vertical: width)
  --os-handle-perpendicular-size-hover: 100%;
  // The :active axis-perpedicular size of the scrollbar handle (horizontal: height, vertical: width)
  --os-handle-perpendicular-size-active: 100%;
  // Increases the interactive area of the scrollbar handle.
  --os-handle-interactive-area-offset: 0;
}

You can alter the properties either for both scrollbars at once or per scrollbar axis. In the example below I've chosen os-theme-custom as the theme name:

// horizontal and vertical scrollbar are 10px 
.os-theme-custom {
  --os-size: 10px;
}

// horizontal scrollbar is 10px
.os-theme-custom.os-scrollbar-horizontal {
  --os-size: 10px;
}
// vertical scrollbar is 20px
.os-theme-custom.os-scrollbar-vertical {
  --os-size: 20px;
}

You can then use your theme by assigning it via the scrollbars.theme option:

OverlayScrollbars(document.body, {
  scrollbars: {
    theme: 'os-theme-custom'
  }
});

Since scrollbar styles are usually simple, this set of options should be enough to get your desired styling. In case you need more freedom you can create your own styles by adding styling to the base class names described in the next section.

Scrollbars structure and CSS class names

The scrollbars HTML markup looks like:

<div class="os-scrollbar os-scrollbar-horizontal">
    <div class="os-scrollbar-track">
        <div class="os-scrollbar-handle">
        </div>
    </div>
</div>
<div class="os-scrollbar os-scrollbar-vertical">
    <div class="os-scrollbar-track">
        <div class="os-scrollbar-handle">
        </div>
    </div>
</div>

The class names are simplified, in a real application the .os-scrollbar element can have additional class names which modify the appearance (mostly visibility and alignment).

Below is a list of the most important class names you will encounter:

If you create your own theme, please only use the classes listed above. All other classes are modifier classes used to change visibility, alignment and pointer-events of the scrollbars.

Gotchas

Its important that the chosen theme class name in your CSS file matches the assigned theme name in the options. If the CSS class name is .my-theme the scrollbars.theme has to be 'my-theme'.

Please be aware of your stack. css-modules for example will alter your class names to prevent naming collisions. Always double check if your CSS is really what you expect it to be.

Plugins are plain objects with a single field, the name of the field is the name of the plugin. This name is the plugins identifier and must be unique across all plugin. In case multiple plugins have the same name, the last added plugin overwrites previously added plugins.

Plugin Modules

A Plugin module is the constructor of a plugin modules instance. There are two kinds of plugin modules: static and instance. A single plugin must have one or more modules. Plugin modules can return an instance, but doesnt have to.

Static Plugin Module

The static plugin module is invoked when the plugin is added with the OverlayScrollbars.plugin function.

Example plugin with a static module:

const staticPlugin = {
  // plugin has the name `examplePlugin`
  examplePlugin: {
    // static function describes a static module and returns the module instance or void / undefined if no instance is needed
    // the `osStatic` parameter is the global `OverlayScrollbars` object
    static: (osStatic) => {
      let count = 0;
      const staticPluginModuleInstance = {
        getCount: () => count,
        increment: () => { count++ },
      }
      return staticPluginModuleInstance;
    }
  }
}

When the plugin is added with the OverlayScrollbars.plugin function, the static module instance is returned:

const staticModuleInstance = OverlayScrollbars.plugin(staticPlugin); // plugins static module is invoked
staticModuleInstance.count; // 0
staticModuleInstance.increment();
staticModuleInstance.count; // 1

Instance Plugin Module

The instance plugin module is invoked when a new OverlayScrollbars instance is created but before the initialized event is dispatched.

Example plugin with a instance module:

const instancePlugin = {
  // plugin has the name `examplePlugin`
  examplePlugin: {
    // instance function describes a instance module and returns the module instance or void / undefined if no instance is needed
    // the `osInstance` parameter is the OverlayScrollbar instance the plugin is bound to
    // the `event` parameter is a function which adds events to the instance which can't be removed from outside the plugin
    // the `osStatic` parameter is the gobal OverlayScrollbar object
    instance: (osInstance, event, osStatic) => {
      let count = 0;

      const instancePluginModuleInstance = {
        getCount: () => count,
        increment: () => { count++ },
      }

      // event which fires when the instance was initialized
      event('initialized', () => {
        console.log("instance initialized");
      });

      // event which fires when the viewport was scrolled
      const removeScrollEvent = event('scroll', () => {
        console.log("viewport scrolled");
        removeScrollEvent(); // removes the event after the first scroll
      });
      
      return instancePluginModuleInstance;
    }
  }
}

When the plugin is added with the OverlayScrollbars.plugin function all OverlayScrollbar instances will add the plugin automatically from that point on. Already created instances will not have the plugin. The instance modules instance is returned with the osInstance.plugin function:

OverlayScrollbars.plugin(instancePlugin); // plugin is added

const osInstance = OverlayScrollbars(document.body, {}); // plugins instance module is invoked
const instancePluginInstance = osInstance.plugin(instancePlugin);

instancePluginInstance.count; // 0
instancePluginInstance.increment();
instancePluginInstance.count; // 1

TypeScript

// Describes a OverlayScrollbar plugin.
export type Plugin<
  // the name of the plugin
  Name extends string = string,
  // the module instance type of the static module
  S extends PluginModuleInstance | void = PluginModuleInstance | void, 
  // the module instance type of the instance module
  I extends PluginModuleInstance | void = PluginModuleInstance | void 
> = {
  [pluginName in Name]: PluginModule;
};

// Describes a OverlayScrollbar plugin which has only a static module.
export type StaticPlugin<
  Name extends string = string,
  T extends PluginModuleInstance = PluginModuleInstance
> = Plugin;

// Describes a OverlayScrollbar plugin which has only a instance module.
export type InstancePlugin<
  Name extends string = string,
  T extends PluginModuleInstance = PluginModuleInstance
> = Plugin;

// Infers the type of the static modules instance of the passed plugin.
export type InferStaticPluginModuleInstance;

// Infers the type of the instance modules instance of the passed plugin.
export type InferInstancePluginModuleInstance;

If you applied OverlayScrollbars to the body element you can use window.scrollX, window.scrollY, window.scroll, window.scrollTo, window.scrollBy or any other native api.

If the plugin was applied to any other element you have to get the viewport element with the instance.elements() function first. With this element you can use element.scrollTop, element.scrollLeft, element.scroll, element.scrollTo, element.scrollBy or any other native api.

const { viewport } = osInstance.elements();
const { scrollLeft, scrollTop } = viewport; // get scroll offset
viewport.scrollTo({ top: 0 }); // set scroll offset

You can adjust a scrollbars handle length by setting a min-width / min-height and max-width / max-height style:

/* horizontal boundaries */
.os-scrollbar-horizontal .os-scrollbar-handle {
  min-width: 50px;
  max-width: 200px;
}
/* vertical boundaries */
.os-scrollbar-vertical .os-scrollbar-handle {
  min-height: 40px;
  max-height: 40px;
}

You can assign the same value to both properties to force the scrollbar to be always the same size.
Setting the width and height properties won't work since those are set by the plugin automatically.