yarn add scroll-percentage

import { calculateScrollPercentage } from 'scroll-percentage'

Or attach this script in your html.

<script src="https://unpkg.com/[email protected]/dist/index.js"></script>

document.querySelector('.parent-elem').addEventListener('scroll',e => {
  let scrollPercentage = calculateScrollPercentage(e.target)
  console.log('scroll percentage', scrollPercentage)
})

// If the parent/scrolling element is `window`, 
// then provide a second parameter - height of the child element.
// Which is basically - the element `window` is scrolling by -

let childElemHeight = document.querySelector('.child-elem').clientHeight

window.addEventListener('scroll', e => {
  let scrollPercentage = calculateScrollPercentage(e.target, childElemHeight)
  console.log('scroll percentage', scrollPercentage)
})

N.B: While calculating the height of child elemenet, adjust the value with margin/padding size.