D3 Examples - I'm lost

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>
​
<body>
  <script>
    // Feel free to change or delete any of the code you see in this editor!
    var svg = d3.select("body").append("svg")
      .attr("width", 2000)
      .attr("height", 1000)
​
   /* svg.append("text")
      .text("Edit the code below to change me!")
      .attr("y", 200)
      .attr("x", 120)
      .attr("font-size", 36)
      .attr("font-family", "monospace")
    */
    
    
   var genes=[{"name": "1", "start": 50, "end":150, "direction": "F", "function":"function1", "fill": "turquoise"},
    {"name": "2", "start": 160, "end":300, "direction": "F", "function":"Function 2", "fill": "skyblue"},
        {"name": "3", "start": 250, "end":420, "direction": "R", "function": "Function 3", "fill": "teal"},
    {"name": "4", "start": 400, "end":500, "direction": "F", "function": "Function 4"}];
    
    var ruler=[{"name":""}]
   
    console.log(genes);
    
    svgEnter= svg.selectAll("rect")
      .data(genes)
        .enter()
        .append("rect")
        .attr("y", function(d) {if (d.direction =="F") {
      return 100;} else {return 250;}})
    
        .attr("x", function(d) {if(d.direction=="F") {return 0 - (d.end - d.start);} else{return 2000;}})
    
      .attr("width", function(d) {return d.end-d.start})
        .attr("height", 30)
        .attr("fill", function (d) {if(d.direction=="F") {return "turquoise";} else{return "salmon";}})     
    
    /*
      .transition()
        .duration(1500)
      .attr("height", 30)
        */
    
     
      .attr("opacity", 1)
    /*
        .transition()
        .duration(1000)
        .delay(function (d, i) {return i * 1000})
        .attr("opacity", 1)
    */
        .transition()
        .duration(2000)
        .attr("x", function (d) {return d.start})
        
   svgEnter.append("text")
    .text(function(d) {return d.name})
    .attr("y", function(d) {if (d.direction =="F") {
      return 100;} else {return 250;}})
    .attr("x", function(d) {if(d.direction=="F") {return 0 - (d.end - d.start);} else{return 2000;}})
   .attr("font-family", "monospace")
   .attr("font-size", 25)
    
   .transition()
        .duration(2000)
        .attr("x", function (d) {return d.start})
   
    svg.selectAll("rect")
    
    ;
    
    //ttr("fill", function(d) {return d.fill})
        //.attr("stroke", "black");
    
​
​
  </script>
</body>
    