back-to-top.js

// Button/Link: #backToTopBtn  

document.getElementById('backToTopBtn').addEventListener('click', () => {
    var start = window.pageYOffset, startTime = null
    let step = (timestamp) => {
      if (!startTime) startTime = timestamp
      var progress = timestamp - startTime,
        percentage = Math.min(progress / 1000, 1)
      window.scrollTo(0, start * (1 - percentage))
      if (progress < 1000) window.requestAnimationFrame(step)
    }
    window.requestAnimationFrame(step)
  })