Plotting points on a map in D3

This is in answer to this Stack Overflow question about how to place points on a map and how projections work.

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Point on a map D3</title>

var width = 950,
    height = 550;

// set projection
var projection = d3.geo.mercator();

// create path variable
var path = d3.geo.path()
    .projection(projection);

d3.json("us.json", function(error, topo) { console.log(topo);

states = topojson.feature(topo, topo.objects.states).features

// set projection parameters
  	projection
      .scale(1000)
      .center([-106, 37.5])

// create svg variable
    var svg = d3.select("body").append("svg")
    				.attr("width", width)
    				.attr("height", height);

// points
    aa = [-122.490402, 37.786453];
	bb = [-122.389809, 37.72728];

console.log(projection(aa),projection(bb));

// add states from topojson
	svg.selectAll("path")
      .data(states).enter()
      .append("path")
      .attr("class", "feature")
      .style("fill", "steelblue")
      .attr("d", path);

// put boarder around states 
  	svg.append("path")
      .datum(topojson.mesh(topo, topo.objects.states, function(a, b) { return a !== b; }))
      .attr("class", "mesh")
      .attr("d", path);

// add circles to svg
    svg.selectAll("circle")
		.data([aa,bb]).enter()
		.append("circle")
		.attr("cx", function (d) { console.log(projection(d)); return projection(d)[0]; })
		.attr("cy", function (d) { return projection(d)[1]; })
		.attr("r", "8px")
		.attr("fill", "red")

});

</script>
    
</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

Modified http://d3js.org/topojson.v1.min.js to a secure url

https://d3js.org/d3.v3.min.js

https://d3js.org/topojson.v1.min.js