HTMLを含んだcontenteditable内のカーソル位置取得

README.md

HTMLを含んだcontenteditable内のカーソル位置取得 DEMO

index.html

Select something in the content below:

<div id="editor" contenteditable="true">A <i>wombat</i> is a marsupial native to <b>Australia</b>
  <p>test</p>
</div>
<div id="selectionLog"></div>

script.js

function getSelectionCharacterOffsetWithin(element) {
    var start = 0;
    var end = 0;
    var doc = element.ownerDocument || element.document;
    var win = doc.defaultView || doc.parentWindow;
    var sel;
    if (typeof win.getSelection != "undefined") {
        sel = win.getSelection();
        if (sel.rangeCount > 0) {
            var range = win.getSelection().getRangeAt(0);
            var preCaretRange = range.cloneRange();
            preCaretRange.selectNodeContents(element);
            preCaretRange.setEnd(range.startContainer, range.startOffset);
          console.log(range.startOffset);
            start = preCaretRange.toString().length;
            preCaretRange.setEnd(range.endContainer, range.endOffset);
            end = preCaretRange.toString().length;
        }
    } else if ( (sel = doc.selection) && sel.type != "Control") {
        var textRange = sel.createRange();
        var preCaretTextRange = doc.body.createTextRange();
        preCaretTextRange.moveToElementText(element);
        preCaretTextRange.setEndPoint("EndToStart", textRange);
        start = preCaretTextRange.text.length;
        preCaretTextRange.setEndPoint("EndToEnd", textRange);
        end = preCaretTextRange.text.length;
    }
    return { start: start, end: end };
}

function reportSelection() {
  var selOffsets = getSelectionCharacterOffsetWithin( document.getElementById("editor") );
  document.getElementById("selectionLog").innerHTML = "Selection offsets: " + selOffsets.start + ", " + selOffsets.end;
}

window.onload = function() {
  document.addEventListener("selectionchange", reportSelection, false);
  document.addEventListener("mouseup", reportSelection, false);
  document.addEventListener("mousedown", reportSelection, false);
  document.addEventListener("keyup", reportSelection, false);
};

style.css

#editor {
  background-color: #eee;
  padding: 1rem;
}

#editor[contenteditable=true]:empty:before {
  content: attr(placeholder);
  color: #999;
  display: block; /* For Firefox */
}