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;}
