title: Uncaught type error on d3.js
body: <p>I am a beginner at d3.js, and am trying to recreate this: <a href="https://bl.ocks.org/mbostock/4062045" rel="nofollow">https://bl.ocks.org/mbostock/4062045</a> with the node and link data in the html file. I have this so far:</p>
  
  <pre><code>  var svg = d3.select("svg"),
        width = +svg.attr("width"),
        height = +svg.attr("height");
  
    var color = d3.scaleOrdinal(d3.schemeCategory20);
  
    var simulation = d3.forceSimulation()
        .force("link", d3.forceLink().id(function(d) { return d.id; }))
        .force("charge", d3.forceManyBody())
        .force("center", d3.forceCenter(width / 2, height / 2));
  
      var link = svg.append("svg:g")
          .attr("class", "links")
        .selectAll("line")
        .data(links)
        .enter().append("line")
          .attr("stroke-width", function(d) { return Math.sqrt(d.value); });
  
      var node = svg.append("svg:g")
          .attr("class", "nodes")
        .selectAll("circle")
        .data(nodes)
        .enter().append("circle")
          .attr("r", 5)
          .attr("fill", function(d) { return color(d.group); })
          .call(d3.drag()
              .on("start", dragstarted)
              .on("drag", dragged)
              .on("end", dragended));
  
      node.append("title")
          .text(function(d) { return d.id; });
  
      simulation
          .nodes(nodes)
          .on("tick", ticked);
  
      simulation.force("link")
          .links(links);
  
      function ticked() {
        link
            .attr("x1", function(d) { return d.source.x; })
            .attr("y1", function(d) { return d.source.y; })
            .attr("x2", function(d) { return d.target.x; })
            .attr("y2", function(d) { return d.target.y; });
  
        node
            .attr("cx", function(d) { return d.x; })
            .attr("cy", function(d) { return d.y; });
      }
  
    function dragstarted(d) {
      if (!d3.event.active) simulation.alphaTarget(0.3).restart();
      d.fx = d.x;
      d.fy = d.y;
    }
  
    function dragged(d) {
      d.fx = d3.event.x;
      d.fy = d3.event.y;
    }
  
    function dragended(d) {
      if (!d3.event.active) simulation.alphaTarget(0);
      d.fx = null;
      d.fy = null;
    }
  </code></pre>
  
  <p>however i get the error index.html:35 Uncaught TypeError: svg.append(...).attr(...).selectAll(...).data(...).enter is not a function. Why is this?</p>
  
  <p>Thanks so much</p>