JSFiddle - React, Tailwind, and code Playground

HTML

<div class='no-key'>
    <div class='a'>already here</div>
</div>
<br/>
<div class='yes-key'>
    <div>already here</div>
</div>

JavaScript

function id(d){ return d; }

d3.select('.no-key').selectAll('div')
    .data([1,2,3])
    .enter().append('div')
    .text(id);

d3.select('.yes-key').selectAll('div')
    .data([1,2,3], id)
    .enter().append('div')
    .text(id);