Pluggable browser scroll management
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README.md

scroll-behavior Travis npm

Pluggable browser scroll management.

This library is not generally meant to be used directly by applications. Instead, it's meant to be used in integrations for routing libraries or frameworks. For examples of such integrations, see:

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Usage

import ScrollBehavior from 'scroll-behavior';

/* ... */

const scrollBehavior = new ScrollBehavior({
  addTransitionHook,
  stateStorage,
  getCurrentLocation,
  /* shouldUpdateScroll, */
});

// After a transition:
scrollBehavior.updateScroll(/* prevContext, context */);

Guide

Installation

$ npm i -S scroll-behavior

Basic usage

Create a ScrollBehavior object with the following arguments:

  • addTransitionHook: this function should take a transition hook function and return an unregister function
    • The transition hook function should be called immediately before a transition updates the page
    • The unregister function should remove the transition hook when called
  • stateStorage: this object should implement read and save methods
    • The save method should take a location object, a nullable element key, and a truthy value; it should save that value for the duration of the page session
    • The read method should take a location object and a nullable element key; it should return the value that save was called with for that location and element key, or a falsy value if no saved value is available
  • getCurrentLocation: this function should return the current location object

This object will keep track of the scroll position. Call the updateScroll method on this object after transitions to emulate the default browser scroll behavior on page changes.

Call the stop method to tear down all listeners.

Custom scroll behavior

You can customize the scroll behavior by providing a shouldUpdateScroll callback when constructing the ScrollBehavior object. When you call updateScroll, you can pass in up to two additional context arguments, which will get passed to this callback.

The callback can return:

  • a falsy value to suppress updating the scroll position
  • a position array of x and y, such as [0, 100], to scroll to that position
  • a string with the id or name of an element, to scroll to that element
  • a truthy value to emulate the browser default scroll behavior

Assuming we call updateScroll with the previous and current location objects:

const scrollBehavior = new ScrollBehavior({
  ...options,
  shouldUpdateScroll: (prevLocation, location) => (
    // Don't scroll if the pathname is the same.
    !prevLocation || location.pathname !== prevLocation.pathname
  ),
});
const scrollBehavior = new ScrollBehavior({
  ...options,
  shouldUpdateScroll: (prevLocation, location) => (
    // Scroll to top when attempting to visit the current path.
    prevLocation && location.pathname === prevLocation.pathname ? [0, 0] : true
  ),
});

Scrolling elements other than window

Call the registerElement method to register an element other than window to have managed scroll behavior. Each of these elements needs to be given a unique key at registration time, and can be given an optional shouldUpdateScroll callback that behaves as above. This method should also be called with the current context per updateScroll above, if applicable, to set up the element's initial scroll position.

scrollBehavior.registerScrollElement(
  key, element, shouldUpdateScroll, context,
);

To unregister an element, call the unregisterElement method with the key used to register that element.

Further scroll behavior customization

If you need to further customize scrolling behavior, subclass the ScrollBehavior class, then override methods as needed. For example, with the appropriate polyfill, you can override scrollToTarget to use smooth scrolling for window.

class SmoothScrollBehavior extends ScrollBehavior {
  scrollToTarget(element, target) {
    if (element !== window) {
      super.scrollToTarget(element, target);
      return;
    }

    if (typeof target === 'string') {
      const targetElement = (
        document.getElementById(target) ||
        document.getElementsByName(target)[0]
      );
      if (targetElement) {
        targetElement.scrollIntoView({ behavior: 'smooth' });
        return;
      }

      // Fallback to scrolling to top when target fragment doesn't exist.
      target = [0, 0]; // eslint-disable-line no-param-reassign
    }

    const [left, top] = target;
    window.scrollTo({ left, top, behavior: 'smooth' });
  }
}

Integrations should accept a createScrollBehavior callback that can create an instance of a custom scroll behavior class.