[javascript] Get contentEditable caret index position

This one builds on @alockwood05's answer and provides both get and set functionality for a caret with nested tags inside the contenteditable div as well as the offsets within nodes so that you have a solution that is both serializable and de-serializable by offsets as well.

I'm using this solution in a cross-platform code editor that needs to get the caret start/end position prior to syntax highlighting via a lexer/parser and then set it back immediately afterward.

function countUntilEndContainer(parent, endNode, offset, countingState = {count: 0}) {
    for (let node of parent.childNodes) {
        if (countingState.done) break;
        if (node === endNode) {
            countingState.done = true;
            countingState.offsetInNode = offset;
            return countingState;
        }
        if (node.nodeType === Node.TEXT_NODE) {
            countingState.offsetInNode = offset;
            countingState.count += node.length;
        } else if (node.nodeType === Node.ELEMENT_NODE) {
            countUntilEndContainer(node, endNode, offset, countingState);
        } else {
            countingState.error = true;
        }
    }
    return countingState;
}

function countUntilOffset(parent, offset, countingState = {count: 0}) {
    for (let node of parent.childNodes) {
        if (countingState.done) break;
        if (node.nodeType === Node.TEXT_NODE) {
            if (countingState.count <= offset && offset < countingState.count + node.length)
            {
                countingState.offsetInNode = offset - countingState.count;
                countingState.node = node; 
                countingState.done = true; 
                return countingState; 
            }
            else { 
                countingState.count += node.length; 
            }
        } else if (node.nodeType === Node.ELEMENT_NODE) {
            countUntilOffset(node, offset, countingState);
        } else {
            countingState.error = true;
        }
    }
    return countingState;
}

function getCaretPosition()
{
    let editor = document.getElementById('editor');
    let sel = window.getSelection();
    if (sel.rangeCount === 0) { return null; }
    let range = sel.getRangeAt(0);    
    let start = countUntilEndContainer(editor, range.startContainer, range.startOffset);
    let end = countUntilEndContainer(editor, range.endContainer, range.endOffset);
    let offsetsCounts = { start: start.count + start.offsetInNode, end: end.count + end.offsetInNode };
    let offsets = { start: start, end: end, offsets: offsetsCounts };
    return offsets;
}

function setCaretPosition(start, end)
{
    let editor = document.getElementById('editor');
    let sel = window.getSelection();
    if (sel.rangeCount === 0) { return null; }
    let range = sel.getRangeAt(0);
    let startNode = countUntilOffset(editor, start);
    let endNode = countUntilOffset(editor, end);
    let newRange = new Range();
    newRange.setStart(startNode.node, startNode.offsetInNode);
    newRange.setEnd(endNode.node, endNode.offsetInNode);
    sel.removeAllRanges();
    sel.addRange(newRange);
    return true;
}