How to access "previous sibling" of `this` when iterating over a selection?

Question

In the following code, assume that $sel is some arbitrary d3.js selection:

$sel.each(function(d, i, j) {
              var prev = ??? ;
              ...
          });

What can I replace the ??? to assign to prev a d3.js selection consisting of the previous sibling of this (if any such exists)?

(If this is a first child, and therefore has no previous sibling, an empty d3.js selection should be assigned to prev.)

Answer 1

It might be as simple as

var prev = d3.select(this.previousSibling);

If you are interested in element nodes only you would go for

var prev = d3.select(this.previousElementSibling);

d3.selectAll(".c").each(function() {
    console.log(d3.select(this));                         // Element <p class="c"></p>
    console.log(d3.select(this.previousSibling));         // text node containing whitespace between b and c
    console.log(d3.select(this.previousElementSibling));  // Element <p class="b"></p>
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>
<div>
    <p class="a"></p>
    <p class="b"></p>
    <p class="c"></p>
    <p class="d"></p>
    <p class="e"></p>
</div>