The Node.previousSibling read-only property returns the node immediately preceding the specified one in its parent's childNodes list, or null if the specified node is the first in that list.
previousNode = node.previousSibling;
// <a><b1 id="b1"/><b2 id="b2"/></a>
alert(document.getElementById("b1").previousSibling); // null
alert(document.getElementById("b2").previousSibling.id); // "b1"
Gecko-based browsers insert text nodes into a document to represent whitespace in the source markup. Therefore a node obtained, for example, using Node.firstChild or Node.previousSibling may refer to a whitespace text node rather than the actual element the author intended to get.
See Whitespace in the DOM and W3C DOM 3 FAQ: Why are some Text nodes empty? for more information.
To navigate the opposite way through the child nodes list use Node.nextSibling.
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https://developer.mozilla.org/en-US/docs/Web/API/Node/previousSibling