D3 Examples - Explanation of data, enter and append & enter/exit props when .data is called

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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; }
    svg {
      width: 100%;
      height: 100%;
    }
  </style>
</head>
​
<body>
  <svg></svg>
</body>
​
<script>
    var rectWidth = 100;
    var height = 300;
    var data = [100 , 250 , 175, 200, 120];
    
    var svg = d3.select('svg');
    var dataSvg = svg.selectAll('rect')
        .data(data);
        
        // '.data(someData)' always returns you an object containing an enter & exit
          // state, which you can manipulate to do your updates!
          console.log(dataSvg);
  
        dataSvg.enter().append('rect')
        .attr('x', (d, i) => i * rectWidth)
        // To get the y axis showing from the bottom to top we
      // substract the data from the height. This is due to 
      // how the coordinate system of svg works
        .attr('y', d => height - d)
        .attr('width', rectWidth)
        .attr('height', d => d)
        .attr('fill', 'blue')
        .attr('stroke', '#fff');
  
      // Shows all the nodes you have
    console.log(enter.nodes());
    // Shows you the data that is being bound
      console.log(enter.data());
  </script>
​
    