gradient path

<!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>

var c = [250, 250]; // center
var r = 200;        // radius
var complete = 0.8; // percent

var circlePath = `
M ${c[0]} ${c[1]-r} 
a ${r},${r} 0 1,0 0, ${(r * 2)} 
a ${r},${r} 0 1,0 0, -${(r * 2)}
Z
`;

var colorInterpolator = d3.interpolateRgbBasis(["rgb(225, 65, 118)", "rgb(108, 125, 255)", "rgb(225, 65, 118)"]);
var colorHandler = (d, i, nodes) => {
  let color = d3.color(colorInterpolator(d.t));
  color.opacity = i/nodes.length > 1-complete ? 1 : 0;
  return color; 
};

var path = d3.select("path").attr("d", circlePath).remove();

console.time("[gradient stroke performance]");

d3.select("svg").selectAll("path")
  .data(quads(samples(path.node(), 8)))
.enter().append("path")
  .style("fill", colorHandler)
  .style("stroke", colorHandler)
  .style("stroke-width", 30)
  .style("stroke-linecap", (d, i, all) => i === 0 || i === all.length ? "round" : "round" )
  .attr("d", d => lineJoin(d[0], d[1], d[2], d[3], 32) );

console.timeEnd("[gradient stroke performance]");
    
    // Sample the SVG path uniformly with the specified precision.
function samples(path, precision) {
  var n = path.getTotalLength(), t = [0], i = 0, dt = precision;
  while ((i += dt) < n) t.push(i);
  t.push(n);
  return t.map(function(t) {
    var p = path.getPointAtLength(t), a = [p.x, p.y];
    a.t = t / n;
    return a;
  });
}

// Compute quads of adjacent points [p0, p1, p2, p3].
function quads(points) {
  return d3.range(points.length - 1).map(function(i) {
    var a = [points[i - 1], points[i], points[i + 1], points[i + 2]];
    a.t = (points[i].t + points[i + 1].t) / 2;
    return a;
  });
}

// Compute stroke outline for segment p12.
function lineJoin(p0, p1, p2, p3, width) {
  var u12 = perp(p1, p2),
      r = width / 2,
      a = [p1[0] + u12[0] * r, p1[1] + u12[1] * r],
      b = [p2[0] + u12[0] * r, p2[1] + u12[1] * r],
      c = [p2[0] - u12[0] * r, p2[1] - u12[1] * r],
      d = [p1[0] - u12[0] * r, p1[1] - u12[1] * r];

if (p0) { // clip ad and dc using average of u01 and u12
    var u01 = perp(p0, p1), e = [p1[0] + u01[0] + u12[0], p1[1] + u01[1] + u12[1]];
    a = lineIntersect(p1, e, a, b);
    d = lineIntersect(p1, e, d, c);
  }

if (p3) { // clip ab and dc using average of u12 and u23
    var u23 = perp(p2, p3), e = [p2[0] + u23[0] + u12[0], p2[1] + u23[1] + u12[1]];
    b = lineIntersect(p2, e, a, b);
    c = lineIntersect(p2, e, d, c);
  }

return "M" + a + "L" + b + " " + c + " " + d + "Z";
}

// Compute intersection of two infinite lines ab and cd.
function lineIntersect(a, b, c, d) {
  var x1 = c[0], x3 = a[0], x21 = d[0] - x1, x43 = b[0] - x3,
      y1 = c[1], y3 = a[1], y21 = d[1] - y1, y43 = b[1] - y3,
      ua = (x43 * (y1 - y3) - y43 * (x1 - x3)) / (y43 * x21 - x43 * y21);
  return [x1 + ua * x21, y1 + ua * y21];
}

// Compute unit vector perpendicular to p01.
function perp(p0, p1) {
  var u01x = p0[1] - p1[1], u01y = p1[0] - p0[0],
      u01d = Math.sqrt(u01x * u01x + u01y * u01y);
  return [u01x / u01d, u01y / u01d];
}

</script>
</body>

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