function mercator(x, y) {
  return [x, Math.log(Math.tan(Math.PI / 4 + y / 2))];
}

<script type="module">

import {geoEqualEarth, geoPath} from "https://cdn.jsdelivr.net/npm/d3-geo@3/+esm";

const projection = geoEqualEarth();
const path = geoPath(projection);

</script>

<script src="https://cdn.jsdelivr.net/npm/d3-array@3"></script>
<script src="https://cdn.jsdelivr.net/npm/d3-geo@3"></script>
<script>

const projection = d3.geoEqualEarth();
const path = d3.geoPath(projection);

</script>

svg.append("path")
    .datum({type: "FeatureCollection", features: features})
    .attr("d", d3.geoPath());

svg.selectAll("path")
  .data(features)
  .enter().append("path")
    .attr("d", d3.geoPath());

var projection = d3.geoTransverseMercator()
    .rotate([74 + 30 / 60, -38 - 50 / 60])
    .fitExtent([[20, 20], [940, 480]], nj);

projection.fitExtent([[0, 0], [width, height]], object);
projection.fitSize([width, height], object);

var mercator = d3.geoProjection(function(x, y) {
  return [x, Math.log(Math.tan(Math.PI / 4 + y / 2))];
});

// y0 and y1 represent two parallels
function conicFactory(phi0, phi1) {
  return function conicRaw(lambda, phi) {
    return […, …];
  };
}

function conicCustom() {
  var phi0 = 29.5,
      phi1 = 45.5,
      mutate = d3.geoProjectionMutator(conicFactory),
      projection = mutate(phi0, phi1);

  projection.parallels = function(_) {
    return arguments.length ? mutate(phi0 = +_[0], phi1 = +_[1]) : [phi0, phi1];
  };

  return projection;
}

function center() {
  return [0, 0];
}

function radius() {
  return 90;
}

function precision() {
  return 6;
}

function geoGraticule10() {
  return d3.geoGraticule()();
}

{
  "type": "Polygon",
  "coordinates": [
    [[0, 0], [0, 1], [1, 1], [1, 0], [0, 0]]
  ]
}

stream.polygonStart();
stream.lineStart();
stream.point(0, 0);
stream.point(0, 1);
stream.point(1, 1);
stream.point(1, 0);
stream.lineEnd();
stream.polygonEnd();

var reflectY = d3.geoTransform({
  point: function(x, y) {
    this.stream.point(x, -y);
  }
});

function matrix(a, b, c, d, tx, ty) {
  return d3.geoTransform({
    point: function(x, y) {
      this.stream.point(a * x + b * y + tx, c * x + d * y + ty);
    }
  });
}