trying to display pop density

<!DOCTYPE html>
<meta charset='utf-8'>
<title>US Counties</title>

// This demo modified slightly from https://bl.ocks.org/mbostock/4108203

var svg = d3.select("svg").append("g")
    .attr('transform', 'scale(4)translate(-700, -100)');

var path = d3.geoPath();

var filename = "https://raw.githubusercontent.com/qaisarmehmood/umbcvis/master/Newyork.population%20Data.csv"

d3.queue()
    .defer(d3.json,"https://unpkg.com/us-atlas@1/us/10m.json")
    .defer(d3.csv, filename)
    .await(ready);

function ready(error, us, csv) {
  if (error) throw error;

console.log('here is the csv data:', csv);

var counties = topojson.feature(us, us.objects.counties).features;
  
  
  
  // Convert topojson to GeoJSON
    geojson = topojson.feature(json, json.objects.tracts);
    tracts = geojson.features;

// Set the projection's scale and translate based on the GeoJSON
    projection.fitSize([960, 600], geojson);

// Extract an array of features (one tract for each feature)
    tracts.forEach(function(tract) {
        var FIPS = tract.properties.COUNTYFP;
        var tractce = tract.properties.TRACTCE;
        pop =  population.filter(function(d) { return (d[2] === countyfips)
                                                   && (d[3] === tractce); });
        pop = +pop[0][0];
        var aland = tract.properties.ALAND / 2589975.2356;

//area in square miles
        tract.properties.density = pop / aland;
    });
    svg.selectAll('path.tract')
        .data(tracts)
      .enter()
        .append('path')
        .attr('class', 'tract')
        .attr('d', path)
        .style('fill', function (d) {return threshold(d.properties.density); })
        .style('stroke', '#000')

// Examine one of the county features in the developer console.
  // The "id" attribute is a 5-digit GEOID (state + county).
  // See: https://www.census.gov/geo/reference/geoidentifiers.html
  console.log(counties[0]);

// Get New York counties
  newyork = counties.filter(function(d) { return d.id.slice(0,2) === "36"; })

svg.selectAll('path.county')
      .data( newyork )
    .enter().append("path")
      .attr("d", path)
      .attr('fill', '#fff')
      .attr('fill-opacity', '0')
      .attr("stroke", "#aaa")
      .attr("stroke-width", 0.5)
      .on('mouseover', function(d) { console.log(d); });

svg.append("path")
      .attr("stroke-width", 0.5)
      .attr("d", path(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; })));

svg.append("path")
      .attr("d", path(topojson.feature(us, us.objects.nation)));
}

</script>

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

https://unpkg.com/topojson-client@3