A straight forward way, that iterates through all the chidren of the contenteditable div until it hits the endContainer. Then I add the end container offset and we have the character index. Should work with any number of nestings. uses recursion.
Note: requires a poly fill for ie to support Element.closest('div[contenteditable]')
https://codepen.io/alockwood05/pen/vMpdmZ
function caretPositionIndex() {
const range = window.getSelection().getRangeAt(0);
const { endContainer, endOffset } = range;
// get contenteditableDiv from our endContainer node
let contenteditableDiv;
const contenteditableSelector = "div[contenteditable]";
switch (endContainer.nodeType) {
case Node.TEXT_NODE:
contenteditableDiv = endContainer.parentElement.closest(contenteditableSelector);
break;
case Node.ELEMENT_NODE:
contenteditableDiv = endContainer.closest(contenteditableSelector);
break;
}
if (!contenteditableDiv) return '';
const countBeforeEnd = countUntilEndContainer(contenteditableDiv, endContainer);
if (countBeforeEnd.error ) return null;
return countBeforeEnd.count + endOffset;
function countUntilEndContainer(parent, endNode, countingState = {count: 0}) {
for (let node of parent.childNodes) {
if (countingState.done) break;
if (node === endNode) {
countingState.done = true;
return countingState;
}
if (node.nodeType === Node.TEXT_NODE) {
countingState.count += node.length;
} else if (node.nodeType === Node.ELEMENT_NODE) {
countUntilEndContainer(node, endNode, countingState);
} else {
countingState.error = true;
}
}
return countingState;
}
}