In radius search with d3.quadtree

Adapted https://bl.ocks.org/mbostock/4343214 for search for points in a radius.

<!DOCTYPE html>
<style>

.point {
  fill: #000;
  fill-opacity: 0.4;
}

.point--scanned {
  fill: orange;
  fill-opacity: 1;
  stroke: orange;
  stroke-width: 3px;
}

.point--selected {
  fill: red;
  fill-opacity: 1;
  stroke: red;
  stroke-width: 5px;
}

.node {
  fill: none;
  stroke: #ccc;
  shape-rendering: crispEdges;
}

.circle-selection {
  fill: rgba(0, 0, 0, 0.2);
}

</style>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height"),
    cx = 100,
    cy = 100,
    searchRadius = 80,
    selected;

var random = Math.random,
    data = d3.range(2500).map(function() { return [random() * width, random() * height]; });

var quadtree = d3.quadtree()
    // .extent([[-1, -1], [width + 1, height + 1]])
    .addAll(data);

svg.selectAll(".node")
  .data(nodes(quadtree))
  .enter().append("rect")
    .attr("class", "node")
    .attr("x", function(d) { return d.x0; })
    .attr("y", function(d) { return d.y0; })
    .attr("width", function(d) { return d.y1 - d.y0; })
    .attr("height", function(d) { return d.x1 - d.x0; });

var point = svg.selectAll(".point")
  .data(data)
  .enter().append("circle")
    .attr("class", "point")
    .attr("cx", function(d) { return d[0]; })
    .attr("cy", function(d) { return d[1]; })
    .attr("r", 2);

svg.append("circle")
    .attr("class", "circle-selection")
    .attr('cx', cx)
    .attr('cy', cy)
    .attr('r', searchRadius);

function inCircle(x, y, cx, cy, radius) {
  return (x - cx) * (x - cx) + (y - cy) * (y- cy) < radius * radius;
}

return !rectCircleIntersect(x1, y1, x2 - x1, y2 - y1, cx, cy, searchRadius);
  });
}

// Collapse the quadtree into an array of rectangles.
function nodes(quadtree) {
  var nodes = [];
  quadtree.visit(function(node, x0, y0, x1, y1) {
    node.x0 = x0, node.y0 = y0;
    node.x1 = x1, node.y1 = y1;
    nodes.push(node);
    return (x1 - x0) < 5;
  });
  return nodes;
}

highlightSearch();

</script>

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