Path animation

Playing with stroke-dasharray and stroke-dashoffset attributes to animate a path.

Line from Mike Bostock’s block. Gradient from Nadieh Bremer’s block.

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <svg>
    <linearGradient id="gradient-rainbow-main" gradientUnits="userSpaceOnUse" x1="0%" y1="0%" x2="60%" y2="0%"><stop offset="0" stop-color="#EFB605"></stop><stop offset="0.06666666666666667" stop-color="#E9A501"></stop><stop offset="0.13333333333333333" stop-color="#E48405"></stop><stop offset="0.2" stop-color="#E34914"></stop><stop offset="0.26666666666666666" stop-color="#DE0D2B"></stop><stop offset="0.3333333333333333" stop-color="#CF003E"></stop><stop offset="0.4" stop-color="#B90050"></stop><stop offset="0.4666666666666667" stop-color="#A30F65"></stop><stop offset="0.5333333333333333" stop-color="#8E297E"></stop><stop offset="0.6" stop-color="#724097"></stop><stop offset="0.6666666666666666" stop-color="#4F54A8"></stop><stop offset="0.7333333333333333" stop-color="#296DA4"></stop><stop offset="0.8" stop-color="#0C8B8C"></stop><stop offset="0.8666666666666667" stop-color="#0DA471"></stop><stop offset="0.9333333333333333" stop-color="#39B15E"></stop><stop offset="1" stop-color="#7EB852"></stop></linearGradient>
  </svg>
  <script>
    var points = [
      [480, 200],
      [580, 400],
      [680, 100],
      [780, 300],
      [180, 300],
      [280, 100],
      [380, 400]
    ];

var svg = d3.select("svg")
      .attr("fill","transparent")
      .attr("stroke","url(#gradient-rainbow-main)")
      .attr("width", 960)
      .attr("height", 500);

var path = svg.append("path")
      .data([points])
      .attr("d", d3.line().curve(d3.curveCardinalClosed))
      .attr("stroke-width",3)
    
  	var totalLength = path.node().getTotalLength();
    
    var visibleLength = 300;
    
    animatePath();
    
    function animatePath(){
      path.attr("stroke-dasharray",visibleLength + " " + totalLength)
        .attr("stroke-dashoffset", visibleLength)
        .transition()
        .ease(d3.easeQuadInOut)
        .duration(2000)
        .attr("stroke-dashoffset", -totalLength)
      	.on("end", animatePath);
    }

</script>
</body>

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