selection.join

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v5.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
    // Feel free to change or delete any of the code you see in this editor!
    var svg = d3.select("body").append("svg")
      .attr("width", 960)
      .attr("height", 500)
    
    var letters = svg.append("g")
    	.attr("transform","translate(200, 200)")

function randomLetters() {
  		return d3.shuffle("abcdefghijklmnopqrstuvwxyz".split(""))
      .slice(0, Math.floor(6 + Math.random() * 20))
      .sort();
		}
  
   const t = letters.transition()
        .duration(750);   
    
  function runLetters(){
    letters.selectAll("text")
      .data(randomLetters(), d => d)
         .join(
        enter => enter.append("text")
            .attr("fill", "green")
            .style("font-size", 36)
      			.style("font-family", "monospace")
            .attr("x", (d, i) => i * 22)
            .attr("y", -30)
            .text(d => d)
          .call(enter => enter.transition(t)
            .attr("y", 0)),
        update => update
            .attr("fill", "black")
            .attr("y", 0)
          .call(update => update.transition(t)
            .attr("x", (d, i) => i * 22)),
        exit => exit
            .attr("fill", "brown")
          .call(exit => exit.transition(t)
            .attr("y", 30)
            .remove())
      );

}  
  runLetters();  
    
  window.setInterval(function (){
    runLetters();
  }, 2500)

</script>
</body>

https://d3js.org/d3.v5.min.js