Attention: this lib isn't very functional at the moment, I don't recommend using it.

svel

Calculate velocity over an interval for scroll or resize events.

Install

npm i svel --save

Usage

svel(scrollY, scrollEvent[, config])

Where scrollY is window.scrollY || window.pageYOffset, scrollEvent is the native event emitted by window.onscroll, and config is an optional config object. Default config: { pool: 10, interval: 100, reset: 50 }.

Basic example:

import svel from 'svel'

window.addEventListener('scroll', event => {
  const speed = svel(window.scrollY, event, {
    pool: 20,
    interval: 100,
    reset: 100
  })

  console.log(speed > 50 ? 'fast!' : 'slow')
})

Ideally you would use a more performant solution that uses a debounce or requestAnimationFrame:

import srraf from 'srraf'
import svel from 'svel'

srrar.scroll.use(({ curr }, event) => {
  const speed = svel(curr, event)

  console.log(speed)
})

How it works

This library is probably overlay simplistic for some use cases.

It tracks average speed from a given number of ticks (pool) and over a specified time interval in milliseconds (interval). It returns this average at each tick, and then you can do what you want with the speed value.

Given the defaults for pool and interval (10 and 100, respectively), you'll get the average speed in pixels/100ms over the last 10 ticks. To get pixels/second, you would change config.interval to 1000.

Example

To run the example, clone this repo, then:

# move into example dir
cd srraf/example
# install deps
npm i
# compile JS
npm run js:build # or js:watch
# serve index.html and update with changes
live-server 

MIT License