更新

正如评论中指出的那样，我的原始答案（如下）仅返回选择的结尾或插入符号的位置。修改代码以返回起始偏移量和结束偏移量非常容易；这是一个这样做的例子：

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 };

}



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);

};


#editor {

  padding: 5px;

  border: solid green 1px;

}


Select something in the content below:



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

<div id="selectionLog"></div>

这是一个函数，将获取指定元素内插入符号的字符偏移量；但是，这是一个幼稚的实现，几乎肯定会与换行符产生不一致，并且不会尝试处理通过CSS隐藏的文本（我怀疑IE会正确地忽略此类文本，而其他浏览器则不会）。正确处理所有这些东西将很棘手。现在我已经尝试过了我的瘦长库。

function getCaretCharacterOffsetWithin(element) {
    var caretOffset = 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.endContainer, range.endOffset);
            caretOffset = 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("EndToEnd", textRange);
        caretOffset = preCaretTextRange.text.length;
    }
    return caretOffset;
}