draw on scroll

custom.js

'use strict'

const path = document.getElementById('line')
const length = path.getTotalLength()
const html = document.querySelector('html')
const height = html.clientHeight
const scrollHeight = html.scrollHeight

path.style.strokeDasharray = length
path.style.strokeDashoffset = length

window.addEventListener('scroll', () => {
  const percentage = this.pageYOffset / (scrollHeight - height)
  const draw = length * percentage
  path.style.strokeDashoffset = length - draw
})

index.html

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <title>JavaScript Draw on Scroll</title>
  <meta name="robots" content="noindex, nofollow">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="styles.css">
</head>

<body>
  <header>
    <h1>JavaScript Draw on Scroll</h1>
  </header>
  <div class="wrapper">
    <div class="container">
      <svg class="scribble" version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 652 560" style="enable-background:new 0 0 652 560;" xml:space="preserve">
      <path id="line" d="M196.5,217.5c0,36.6-117.6,41.9-124,78C63,349.1,204,462,209.5,464.5c51,23,195-42,223-76s110-103,23-192
      	s-117-159-172-151s101,95,72,209s-49,143-151,131s-211-292-85-253s158,163,258,192s370-27,223-117s-145-355-247-98s71,223-123,319
      	s-268,91-213-21s32-233,132-239s308,271,342,290s61.6,43.4,84,38c52.7-12.8,139-155-5-217s-105,11-266,130s-251,269-189-9
      	s-38-387,83-353"/>
      </svg>
    </div>
    <!-- .container-->
  </div>
  <!-- .wrapper-->
  <footer>
    <small><a href="https://opensource.org/licenses/MIT">MIT</a></small>
  </footer>
  <script src="custom.js"></script>
</body>

</html>

styles.css

body {
  background: #eeeeee;
  font-size: 16px;
  font-family: "Trebuchet MS", Helvetica, sans-serif;
  margin: 0;
}

a {
  color: crimson;
  text-decoration: none;
}

a:hover {
  color: red;
}


/* Layout
------------------------------------------*/

header, .container, footer {
  margin: 0 auto;
  width: 88%;
  max-width: 1080px;
}

.wrapper {
  padding: 1em 0;
  background: #ffffff;
  min-height:900px;
}

/* Animated path
------------------------------------------*/

.scribble {
  max-width:500px;
  position:fixed;
  top:20;
}

#line {
  fill: none;
  stroke: red;
  stroke-width: 2px;
}