如何使用Range Object和window.getSelection()获得与body相关的字符偏移量

我正在用Javascript编写一个客户端应用程序，我使用以下函数:

function creation() {
var userSelection;
if (window.getSelection) {
    userSelection = window.getSelection();
}
else if (document.selection) { // should come last; Opera!
    userSelection = document.selection.createRange();
}
var rangeObject = getRangeObject(userSelection);
var startOffset = rangeObject.startOffset;
var endOffset = rangeObject.endOffset;
var startCon = rangeObject.startContainer;
var endCon = rangeObject.endContainer;
var myRange = document.createRange();
myRange.setStart(startCon,rangeObject.startOffset);
myRange.setEnd(endCon, rangeObject.endOffset);
$('#result').text(myRange.toString());
}
function getRangeObject(selectionObject) {
if (selectionObject.getRangeAt) {
    var ret = selectionObject.getRangeAt(0);
    return ret;
}
else { // Safari!
    var range = document.createRange();
    range.setStart(selectionObject.anchorNode, selectionObject.anchorOffset);
    range.setEnd(selectionObject.focusNode, selectionObject.focusOffset);
    return range;
     }
}

我需要一种方法来知道与body元素相关的字符偏移量。我发现了一个函数，可以对元素中的字符进行计数:

function getCharacterOffsetWithin(range, node) {
var treeWalker = document.createTreeWalker(
        node,
        NodeFilter.SHOW_TEXT,
        function (node) {
            var nodeRange = document.createRange();
            nodeRange.selectNode(node);
            return nodeRange.compareBoundaryPoints(Range.END_TO_END, range) < 1 ?
                    NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_REJECT;
        },
        false
);
var charCount = 0;
while (treeWalker.nextNode()) {
    charCount += treeWalker.currentNode.length;
}
if (range.startContainer.nodeType == 3) {
    charCount += range.startOffset;
}
return charCount;
}