Next.js smooth scroll

LinkSmoothScroll.js

import React, { Children } from 'react'
import Router from 'next/router'
import smoothScroll from '../utils/smoothScroll'

// this HOC is taken from https://github.com/zeit/next.js/blob/master/lib/link.js and modified
export default class LinkSmoothScroll extends React.Component {
  constructor (props) {
    super(props)
    this.linkClicked = this.linkClicked.bind(this)
  }
  linkClicked (e) {
    e.preventDefault()
    Router
      .push(this.props.href)
      .then(() => {
        return smoothScroll(this.props.href)
      })
      .then(() => {
        this.props.done && this.props.done()
      })
      .catch(err => {
        this.props.onError && this.props.onError(err)
        console.error(err)
      })
  }
  render () {
    let { children } = this.props
    if (typeof children === 'string') {
      children = <a>{children}</a>
    }
    const child = Children.only(children)
    const props = { onClick: this.linkClicked }
    if (child.type === 'a' && !('href' in child.props)) {
      props.href = this.props.href
    }
    return React.cloneElement(child, props)
  }
}

NavLinks.js

import LinkSmoothScroll from './LinkSmoothScroll'

export default () => (
  <ul className='nav__list'>
    <li className='nav__listItem'>
      <LinkSmoothScroll href='/#home'>
        <a className='link'>Home</a>
      </LinkSmoothScroll>
    </li>
    <li className='nav__listItem'>
      <LinkSmoothScroll href='/#subjects'>
        <a className='link'>subjects</a>
      </LinkSmoothScroll>
    </li>
    <li className='nav__listItem'>
      <LinkSmoothScroll href='/#services'>
        <a className='link'>Services</a>
      </LinkSmoothScroll>
    </li>
    <style jsx>
      {`
        .link {
          text-decoration: none;
          text-transform: uppercase;
          color: #999;
          transition: all 0.3s ease-in-out;
          position: relative;
          font-size: 15px;
        }
        .link::after {
          content: '';
          position: absolute;
          left: 0;
          bottom: -10px;
          width: 0;
          height: 3px;
          background: #23a5cf;
          transition: all 0.3s ease-in-out;
        }
        .link:hover {
          color: #23a5cf;
        }
        .link:hover::after {
          width: 100%;
        }
        .nav__list {
          display: flex;
          align-items: center;
          padding: 0;
          margin: 0;
          list-style-type: none;
        }
        .nav__listItem {
          margin: 0 20px;
        }
        @media (max-width: 950px) {
          .nav__list {
            display: none;
          }
        }
      `}
    </style>
  </ul>
)

smoothScroll.js

// Get the top position of an element in the document
const getTop = function (element, start) {
  // return value of html.getBoundingClientRect().top ... IE : 0, other browsers : -pageYOffset
  if (element.nodeName === 'HTML') return -start
  return element.getBoundingClientRect().top + start
}
// ease in out function thanks to:
// http://blog.greweb.fr/2012/02/bezier-curve-based-easing-functions-from-concept-to-implementation/
const easeInOutCubic = function (t) {
  return t < 0.5 ? 4 * t * t * t : (t - 1) * (2 * t - 2) * (2 * t - 2) + 1
}

// calculate the scroll position we should be in
// given the start and end point of the scroll
// the time elapsed from the beginning of the scroll
// and the total duration of the scroll (default 500ms)
const position = function (start, end, elapsed, duration) {
  if (elapsed > duration) return end
  return start +
    (end - start) *
      easeInOutCubic(
        elapsed / duration
      ) // <-- you can change the easing funtion there
  // return start + (end - start) * (elapsed / duration); // <-- this would give a linear scroll
}

// we use requestAnimationFrame to be called by the browser before every repaint
// if the first argument is an element then scroll to the top of this element
// if the first argument is numeric then scroll to this location
// if the callback exist, it is called when the scrolling is finished
// if context is set then scroll that element, else scroll window
const smoothScroll = function (el, duration, callback, context) {
  duration = duration || 500
  context = context || window
  const start = context.scrollTop || window.pageYOffset
  let end
  if (typeof el === 'number') {
    end = parseInt(el) - 60
  } else {
    end = getTop(el, start) - 60
  }

  const clock = Date.now()
  const requestAnimationFrame = window.requestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    function (fn) {
      window.setTimeout(fn, 15)
    }

  const step = function () {
    const elapsed = Date.now() - clock
    if (context !== window) {
      context.scrollTop = position(start, end, elapsed, duration)
    } else {
      window.scroll(0, position(start, end, elapsed, duration))
    }

    if (elapsed > duration) {
      if (typeof callback === 'function') {
        callback(el)
      }
    } else {
      requestAnimationFrame(step)
    }
  }
  step()
}
export default url => {
  return new Promise(function (resolve, reject) {
    const pattern = /^(\/#.+)|(.+(\/#.+))$/
    if (pattern.test(url)) {
      const hash = pattern.exec(url).filter(item => item).pop()
      const id = hash.replace(/\/?#/, '')
      const el = document.getElementById(id)
      if (el) {
        smoothScroll(el, 600, resolve)
        return
      }
    }
    reject(new Error('Error: hash in URL is invalid or element not found!'))
  })
}