Error displaying geojson features
<p>When I open the index.html from local path "file://desktop/index.html", it shows correctly. 
But when I open the file by node.js in "localhost:8080", I got an error: "TypeError: null is not an object (evaluating 'usStates.features')" 
Can someone help me?</p>

<pre><code> d3.json("us.json", function(usStates) {
                  console.log(usStates.features);
      svg.selectAll("path")
               .data(usStates.features)
               .enter()
               .append("path")
               .attr("d", path)
               .style("stroke", "#fff")
               .style("stroke-width", "0.4")
               .style("fill", "rgb(213,222,217)" )
                   .on("mouseover", function(d) { 
                   var name = d.properties.name;     
</code></pre>

<p><img src="https://i.stack.imgur.com/Dr7GK.png" alt="enter image description here"></p>

<p><a href="https://i.stack.imgur.com/fRnhm.png" rel="nofollow noreferrer">enter image description here</a></p>

<p>Here is my server setting. I think that might be the problem, but I don't know how to fix it.</p>

<pre><code>var app = require('express')();
var server = require('http').createServer(handler);
var io = require('socket.io')(server);
var fs = require('fs');
var Twit = require('twit');
var config = require('./config/config.json');
</code></pre>