Animating along a path

Moving elements along a path while maintaining orientation. For each arrow, the translate is determined by interpolating a point along the length of the path and the rotation is determined by calculating the approximate tangent of the path using two nearby points.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <style>

path {
      fill: dodgerblue;
      stroke: none;
    }

.track {
      fill: none;
      stroke: dodgerblue;
      stroke-width: 3px;
    }

</style>
</head>
<body>
<svg version="1.1" xmlns="https://www.w3.org/2000/svg" xmlns:xlink="https://www.w3.org/1999/xlink" width="960" height="500">
  <defs>
    <path id="arrow" d="M-5,0 L-15,15 L15,0 L-15,-15 Z"/>
  </defs>
  <path class="track" d="M636.5,315c-0.4-18.7,1.9-27.9-5.3-35.9
  	c-22.7-25-107.3-2.8-118.3,35.9c-7,24.4,20.6,37.2,16,71c-4,29.6-30.8,60.7-56.5,61.1c-30.8,0.4-32.9-43.8-81.7-70.2
  	c-50.9-27.6-110.1-12.9-125.2-9.2c-66.1,16.4-82.2,56.9-109.2,47.3c-38-13.6-55.9-112.1-19.8-143.5c39-34,121.2,27.7,148.1-3.8
  	c18-21.1,3.1-74.3-25.2-105.3c-31.1-34.1-70.1-32.4-105.3-76.3c-8.2-10.2-16.9-23.8-15.3-39.7c1.2-11.4,7.5-23.3,15.3-29
  	c33.8-25,101.6,62.6,193.1,59.5c40.1-1.3,38.7-18.5,99.2-38.9c126.2-42.6,242.4-4.9,297.7,13c54.7,17.7,105.4,35,129.8,82.4
  	c13,25.3,22.9,67.7,4.6,87c-11.6,12.3-25.1,5.1-46.6,20.6c-2.8,2-28.9,21.4-32.1,49.6c-3.1,27.4,18.7,35,29,70.2
  	c8.8,30.1,8.5,77.8-18.3,99.2c-32.3,25.8-87,0.6-100-5.3c-69.6-32-67.2-88.4-73.3-109.2z"/>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var path = document.querySelector(".track"),
    totalLength = path.getTotalLength();

var markers = d3.select("svg").selectAll("g")
  .data(d3.range(0, 1, 0.1))
  .enter()
  .append("g")
    .attr("transform",function(d){
      var p = pointAtLength(totalLength * d);
      return "translate(" + p + ") rotate( " + angleAtLength(totalLength * d) + ")";
    });

markers.append("use")
  .attr("xlink:href", "#arrow");

d3.timer(function(t){

markers.attr("transform",function(d){

var pos = (d + t / 15000) * totalLength % totalLength;
    return "translate(" + pointAtLength(pos) + ") rotate( " + angleAtLength(pos) + ")";

});

function pointAtLength(l) {

var xy = path.getPointAtLength(l);
  return [xy.x, xy.y];

}

// Approximate tangent
function angleAtLength(l) {

var a = pointAtLength(Math.max(l - 0.01,0)), // this could be slightly negative
      b = pointAtLength(l + 0.01); // browsers cap at total length

return Math.atan2(b[1] - a[1], b[0] - a[0]) * 180 / Math.PI;

}

</script>

https://d3js.org/d3.v4.min.js