By adding a key to the data-join, letters that are already displayed are put in the update selection. Now updates can occur anywhere in the array, depending on the overlap between the old letters and the new letters. The text content only needs to be set on enter because the mapping from letter to element never changes; however, the x-position of the text element must now be recomputed on update as well as enter.
It'll be easier to see what's going on when we add animated transitions next!
Next: Update Transitions
Previous: General Update Pattern
xxxxxxxxxx<meta charset="utf-8"><style>text { font: bold 48px monospace;}.enter { fill: green;}.update { fill: #333;}</style><svg width="960" height="500"></svg><script src="https://d3js.org/d3.v4.min.js"></script><script>var alphabet = "abcdefghijklmnopqrstuvwxyz".split("");var svg = d3.select("svg"), width = +svg.attr("width"), height = +svg.attr("height"), g = svg.append("g").attr("transform", "translate(32," + (height / 2) + ")");function update(data) { // DATA JOIN // Join new data with old elements, if any. var text = g.selectAll("text") .data(data, function(d) { return d; }); // UPDATE // Update old elements as needed. text.attr("class", "update"); // ENTER // Create new elements as needed. // // ENTER + UPDATE // After merging the entered elements with the update selection, // apply operations to both. text.enter().append("text") .attr("class", "enter") .attr("dy", ".35em") .text(function(d) { return d; }) .merge(text) .attr("x", function(d, i) { return i * 32; }); // EXIT // Remove old elements as needed. text.exit().remove();}// The initial display.update(alphabet);// Grab a random sample of letters from the alphabet, in alphabetical order.d3.interval(function() { update(d3.shuffle(alphabet) .slice(0, Math.floor(Math.random() * 26)) .sort());}, 1500);</script> https://d3js.org/d3.v4.min.js