Self-drawing arrow

Animating a path with an arrowhead using stroke-dasharray and interpolation along the path for the arrow positioning. The translate is determined by 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.

See also: Animating along a path

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

path {
      fill: none;
      stroke: black;
      stroke-width: 2px;
      display: none;
    }

.arrow {
      fill: black;
      stroke: none;
    }

</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">
  <path class="swoop" d="M36.2,268.1C70.2,153.8,108.1,58,140.8,61.2
  	c63.1,6.1,69,376.4,103.1,376.2c33-0.2,47.6-346.4,83.8-346.9c32.4-0.4,54.9,275.7,85.4,274.6c27.7-1,34.1-230,62.3-230.8
  	c26.2-0.7,48.4,195.5,70,193.8c19.1-1.4,20.7-162.2,43.8-158.5c13.8,2.2,19.3,29.9,24,40.2c17.2,37.4,55.8,58.7,97,54.6
  	c13.1-1.3,28.6-6.8,38.9-15c9.4-7.5,15.6-20.4,11.6-31.7c-2.8-7.8-9.8-13.5-17.4-16.9c-9.7-4.4-21.1-5.7-31.2-2
  	c-16.6,6.1-25.2,26.6-14.8,41.8c10.9,16.1,31,17.1,48,21.7c55,14.6,112,3.9,165.4-12.2"/>
  <path class="arrow" d="M-5,0 L-15,15 L15,0 L-15,-15 Z"/>
</svg>

var swoop = d3.select(".swoop"),
    arrow = d3.select(".arrow"),
    path = swoop.node(),
    totalLength = path.getTotalLength();

draw(true);

function draw(immediate) {

var t = d3.transition()
      .duration(4000)
      .delay(immediate ? 0 : 1000)
      .on("start",function(){
        d3.selectAll("path").style("display", "block");
      })
      .on("end",draw);

arrow.transition(t)
    .attrTween("transform",function(){
      return function(t){
        var pos = t * totalLength;
        return "translate(" + pointAtLength(pos) + ") rotate( " + angleAtLength(pos) + ")";
      };
    });

swoop.transition(t)
    .attrTween("stroke-dasharray",function(){
      return d3.interpolateString("0," + totalLength,totalLength + "," + totalLength);
    });

}

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