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                <input id="search" placeholder="Search" />

<h1>Table of contents</h1>
  <li><a class="link-to-section" href="#section-1">Section One</a></li>
  <li><a class="link-to-section" href="#section-2">Section Two</a></li>
  <li><a class="link-to-section" href="#section-3">Section Three</a></li>
  <li><a class="link-to-section" href="#section-4">Section Four</a></li>

<h1 id="section-1" class="section-header">
  Section One
  <button class="back-to-top">Back to top</button>

<h1 id="section-2" class="section-header">
  Section Two
  <button class="back-to-top">Back to top</button>

<h1 id="section-3" class="section-header">
  Section Three
  <button class="back-to-top">Back to top</button>

<h1 id="section-4" class="section-header">
  Section Four
  <button class="back-to-top">Back to top</button>



                body, p, h1, h2 { margin: 0; }

body {
  font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";
  color: #333;
  font-size: 18px;
  background: white;
  padding-bottom: 2000px;

#search {
  display: block;
  width: 300px;
  padding: 10px;
  font-size: 20px;
  margin: 20px auto;

.back-to-top {
  padding-left: 10px;
  border: none;
  text-decoration: underline;
  background: none;

.section-header {
  margin-top: 600px;



/*  -------------------------  */
/*  --  The ease function  --  */
/*  -------------------------  */

function ease({
  startValue = 0,
  endValue = 1,
  durationMs = 200,
  onComplete = () => {},
}) {
  const raf = window.requestAnimationFrame || (func => window.setTimeout(func, 16));
  const stepCount = durationMs / 16;
  const valueIncrement = (endValue - startValue) / stepCount;
  const sinValueIncrement = Math.PI / stepCount;
  let currentValue = startValue;
  let currentSinValue = 0;
  function step() {
    currentSinValue += sinValueIncrement;
    currentValue += valueIncrement * (Math.sin(currentSinValue) ** 2) * 2;
    if (currentSinValue < Math.PI) {
    } else {


/*  ---------------------------  */
/*  --  Some usage examples  --  */
/*  ---------------------------  */

// This will scroll from the current position down to the element with a particular ID
document.querySelectorAll('.link-to-section').forEach(el => el.addEventListener('click', e => {
  // Side-note: NodeList.forEach() isn't supported everywhere
  const targetId ='href');
  const targetEl = document.querySelector(targetId);
  const targetPos = targetEl.getBoundingClientRect().top + window.scrollY;
    startValue: window.scrollY,
    endValue: targetPos,
    durationMs: 400,
    onStep: value => window.scroll(0, value),

// This will smoothly scroll from the current window position to the top.
// When at the top, it will focus the search box and remove the hash from the URL
document.querySelectorAll('.back-to-top').forEach(el => el.addEventListener('click', () => {
    startValue: window.scrollY,
    endValue: 0,
    onStep: value => window.scroll(0, value),
    onComplete: () => {
      // You wouldn't want to do these things before scrolling was finsihed,
      // they would snap the scroll immediately to the top of the page
      document.location.hash = '';