CODE EXAMPLE FOR JAVASCRIPT

js get selection start from contenteditable

/**
  * @param {object} element HTMLElement
  */
function getCaretIndex(element) {
  let position = 0;
  const isSupported = typeof window.getSelection !== "undefined";
  if (isSupported) {
    const selection = window.getSelection();
    if (selection.rangeCount !== 0) {
      const range = window.getSelection().getRangeAt(0);
      const preCaretRange = range.cloneRange();
      preCaretRange.selectNodeContents(element);
      preCaretRange.setEnd(range.endContainer, range.endOffset);
      position = preCaretRange.toString().length;
    }
  }
  return position + 1;
}