W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  height: 2000px;
  background: #f1f1f1;
}

#mySVG {
  position: fixed;
  top: 15%;
  width: 400px;
  height: 210px;
  margin-left:-50px;
}
</style>
</head>
<body>

<h2>Scroll down this window to draw a triangle.</h2>
<p>Scroll back up to reverse the drawing.</p>

// The start position of the drawing
triangle.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
triangle.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
var scrollpercent = (document.body.scrollTop + document.documentElement.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);

var draw = length * scrollpercent;
  
  // Reverse the drawing (when scrolling upwards)
  triangle.style.strokeDashoffset = length - draw;
}
</script>

</body>
</html>