D3 Examples - how enter-update-exit works

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { 
      margin-left:15px;
      position:fixed;
      top:0;right:0;bottom:0;left:0;
      margin-top:15px;
    }
​
    .enter {
      fill: green;
    }
​
    .update {
      fill: red;
    }
​
    .exit {
      fill: brown;
    }
  </style>
</head>
​
<body>
  <script>
    // Feel free to change or delete any of the code you see in this editor!
    var scale = d3.scaleLinear().domain([1, 10]).range([0, 300]); // Pixel space
​
    var svg = d3.select("body").append("svg").attr("width",  350).attr("height", 250);
​
      function render(data){
            var t = d3.transition()
            .duration(750);
​
        var rects = svg.selectAll("rect").data(data, d => d);
​
        rects.enter()
          .append("rect")
              .attr("class", "enter")
          .attr('x', scale)
          .attr("y", 60)
          .attr("width",  20)
          .attr("height", 20)
              .style("fill-opacity", 1e-6)
        .transition(t)
          .attr("y", 0)
          .style("fill-opacity", 1)
​
        rects
              .attr("class", "update")
          .style("fill-opacity", 1)
        .transition(t);
​
        rects.exit()
              .attr("class", "exit")
        .transition(t)
          .attr("y", 60)
          .style("fill-opacity", 1e-6)
          .remove();
              }
​
        let range = (num) => {
        return new Array(num).fill(0).map((_, id) => id + 1);
      }
      
      d3.interval(() => {
            render(range(Math.floor(Math.random() * 10) + 1));
            }, 1000);
​
  </script>
</body>
​
    