D3 Examples - update & exit

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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: 300px
    }
  </style>
</head>
​
<body>
  <svg></svg>
  <script>
        const rectWidth = 50;
    const height = 300;
    const data = [
      [100, 250, 175, 200, 120],
      [230, 120, 300, 145, 75, 250],
      [240, 250, 100]
    ];
    
    const colors = d3.scaleOrdinal(d3.schemeCategory10);
    const svg = d3.select('svg');
    
    function updateBars(data) {
      let bars = svg.selectAll('rect')
        .data(data, d => d); // key function
​
      // exit
      bars.exit().remove();
​
      // enter
      const 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)
        .attr('fill', d => colors(d))
        .transition()
          .attr('y', d => height - d)
        .attr('height', d => d);
    }
​
    updateBars(data[0]);
​
    let index = 2;
    setInterval(() => {
      updateBars(data[index % 3]);
      index += 1;
    }, 2000);
  </script>
</body>
​
    