Transitions: with basic update pattern

<!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>
  <script>
		var rectWidth = 100;
    var height = 300;
    var data = [
      [100, 250, 175, 200, 120],
      [230, 120, 300, 145, 75, 250],
      [240, 250, 100]
    ];

var colors = d3.scaleOrdinal(d3.schemeCategory10);
    var svg = d3.select('svg');
    
    function updateBars(data) {
      var t = d3.transition().duration(2000);
      
      var bars = svg.selectAll('rect')
        .data(data, d => d);

// exit
      bars.exit().transition(t)
				.attr('y', height)
        .attr('height', 0)
        .remove();

// enter
      var enter = bars.enter().append('rect')
        .attr('width', rectWidth)
        .attr('stroke', '#fff')
      	.attr('y', height)
        .attr('x', (d, i) => i * rectWidth);
      // enter + update
      bars = enter.merge(bars)
        .attr('fill', d => colors(d))
      	.transition(t)
        .attr('x', (d, i) => i * rectWidth)
        .attr('y', d => height - d)
        .attr('height', d => d);
    }

var index = 0;
    setInterval(() => {
      updateBars(data[index % 3]);
      index += 1;
    }, 2000);
  </script>
</body>

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