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Scroll Drawing
<h1>Scroll-to-draw</h1>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100.6 107.6" id="star-svg">
<path fill="none" stroke="white" stroke-width="2" id="star-path" d="M43.7,65.8L19.9,83.3c-2.9,1.9-5.1,3.2-7.9,3.2c-5.7,0-10.5-5.1-10.5-10.8
c0-4.8,3.8-8.2,7.3-9.8l27.9-12L8.8,41.4c-3.8-1.6-7.3-5.1-7.3-9.8c0-5.7,5.1-10.5,10.8-10.5c2.9,0,4.8,1,7.6,3.2l23.8,17.4
l-3.2-28.2c-1-6.7,3.5-12,9.8-12c6.3,0,10.8,5.1,9.8,11.7L57,41.8l23.8-17.4c2.9-2.2,5.1-3.2,7.9-3.2c5.7,0,10.5,4.8,10.5,10.5
c0,5.1-3.5,8.2-7.3,9.8L63.9,53.8l27.9,12c3.8,1.6,7.3,5.1,7.3,10.1c0,5.7-5.1,10.5-10.8,10.5c-2.5,0-4.8-1.3-7.6-3.2L57,65.8
l3.2,28.2c1,6.7-3.5,12-9.8,12c-6.3,0-10.8-5.1-9.8-11.7L43.7,65.8z"/>
</svg>
// Get a reference to the <path>
var path = document.querySelector('#star-path');
// Get length of path... ~577px in this case
var pathLength = path.getTotalLength();
// Make very long dashes (the length of the path itself)
path.style.strokeDasharray = pathLength + ' ' + pathLength;
// Offset the dashes so the it appears hidden entirely
path.style.strokeDashoffset = pathLength;
// Jake Archibald says so
// https://jakearchibald.com/2013/animated-line-drawing-svg/
path.getBoundingClientRect();
// When the page scrolls...
window.addEventListener("scroll", function(e) {
// What % down is it?
// https://stackoverflow.com/questions/2387136/cross-browser-method-to-determine-vertical-scroll-percentage-in-javascript/2387222#2387222
// Had to try three or four differnet methods here. Kind of a cross-browser nightmare.
var scrollPercentage = (document.documentElement.scrollTop + document.body.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);
// Length to offset the dashes
var drawLength = pathLength * scrollPercentage;
// Draw in reverse
path.style.strokeDashoffset = pathLength - drawLength;
// When complete, remove the dash array, otherwise shape isn't quite sharp
// Accounts for fuzzy math
if (scrollPercentage >= 0.99) {
path.style.strokeDasharray = "none";
} else {
path.style.strokeDasharray = pathLength + ' ' + pathLength;
}
});

Scroll Drawing

As you scroll down, the % scrolled of the document is calculated and that same % of an SVG <path> is drawn.

A Pen by Chris Coyier on CodePen.

License.

body {
/* feel free to change height */
height: 5000px;
background: linear-gradient(
to bottom,
orange,
darkblue
);
}
h1 {
font-family: "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif;
font-weight: 300;
color: white;
text-transform: uppercase;
text-align: center;
font-size: 22px;
letter-spacing: 5px;
font-weight: 100;
padding: 25px 15px;
text-shadow: 1px 1px 1px #333;
}
#star-svg {
position: fixed;
top: 50%;
left: 50%;
width: 150px;
height: 150px;
margin: -75px 0 0 -75px;
}
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