How to Accurately Determine Character Offsets Within an HTML Element?

Calculating Range Start and End Offsets Relative to Parent Container

Suppose you encounter HTML elements like the one below:

<div>

And a user selects the text "home" with their mouse. Determining the character offsets within the "#parent" element is a common task, especially when selecting HTML tags.

While the "range.startOffset" property provides an offset relative to the immediate container, it's limited to character offsets when the container is a text node. To overcome this limitation, we present an updated solution:

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);
      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 };
}

To demonstrate the updated method, we initialize event listeners to capture selection changes and update a log element on the page:

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);
};

This code enables you to accurately determine the character offsets of a user's selection within an HTML element, even when selecting HTML tags.

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