exit, enter, update, merge Notes

<!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>
    /*
    exit() = stuff being removed
    enter() = new stuff
    update() = stuff staying on the screen (what data() returns)
    merge() enter().merge(returned by .data())
    */
    var width = 900
    var height = 400
    var rectWidth = 24
    
    var data = [0, 1, 3, 5, 11, 13, 28, 29, 35, 37, 58]
    
    var colors = d3.scaleOrdinal(d3.schemeCategory10);
    
    var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height)
    
    // data
    var bars = svg.selectAll('rect')
    	.data(data, d => d)
    
    // exit
    bars.exit().remove()
    
    // enter
    var enter = bars.enter().append('rect')
    	.attr('width', rectWidth)
    	.attr('stroke', '#fff')
    
    // enter + update
    bars = enter.merge(bars)
    	.attr('x', (d, i) => i * rectWidth)
    	.attr('y', d => height - d)
    	.attr('height', d => d)
    	.attr('fill', d => colors(d))

</script>
</body>

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