how enter-update-exit works

<!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>

https://d3js.org/d3.v4.min.js