如何在input extarea元素中接收选定文本的坐标

下面的代码是该函数在此答案处的精确副本。有关代码的解释，请查看链接的答案。演示:http://jsfiddle.net/56Rep/5/

// @author Rob W       https://stackoverflow.com/users/938089/rob-w
// @name               getTextBoundingRect
// @param input          Required HTMLElement with `value` attribute
// @param selectionStart Optional number: Start offset. Default 0
// @param selectionEnd   Optional number: End offset. Default selectionStart
// @param debug          Optional boolean. If true, the created test layer
//                         will not be removed.
function getTextBoundingRect(input, selectionStart, selectionEnd, debug) {
    // Basic parameter validation
    if(!input || !('value' in input)) return input;
    if(typeof selectionStart == "string") selectionStart = parseFloat(selectionStart);
    if(typeof selectionStart != "number" || isNaN(selectionStart)) {
        selectionStart = 0;
    }
    if(selectionStart < 0) selectionStart = 0;
    else selectionStart = Math.min(input.value.length, selectionStart);
    if(typeof selectionEnd == "string") selectionEnd = parseFloat(selectionEnd);
    if(typeof selectionEnd != "number" || isNaN(selectionEnd) || selectionEnd < selectionStart) {
        selectionEnd = selectionStart;
    }
    if (selectionEnd < 0) selectionEnd = 0;
    else selectionEnd = Math.min(input.value.length, selectionEnd);
    // If available (thus IE), use the createTextRange method
    if (typeof input.createTextRange == "function") {
        var range = input.createTextRange();
        range.collapse(true);
        range.moveStart('character', selectionStart);
        range.moveEnd('character', selectionEnd - selectionStart);
        return range.getBoundingClientRect();
    }
    // createTextRange is not supported, create a fake text range
    var offset = getInputOffset(),
        topPos = offset.top,
        leftPos = offset.left,
        width = getInputCSS('width', true),
        height = getInputCSS('height', true);
        // Styles to simulate a node in an input field
    var cssDefaultStyles = "white-space:pre;padding:0;margin:0;",
        listOfModifiers = ['direction', 'font-family', 'font-size', 'font-size-adjust', 'font-variant', 'font-weight', 'font-style', 'letter-spacing', 'line-height', 'text-align', 'text-indent', 'text-transform', 'word-wrap', 'word-spacing'];
    topPos += getInputCSS('padding-top', true);
    topPos += getInputCSS('border-top-width', true);
    leftPos += getInputCSS('padding-left', true);
    leftPos += getInputCSS('border-left-width', true);
    leftPos += 1; //Seems to be necessary
    for (var i=0; i<listOfModifiers.length; i++) {
        var property = listOfModifiers[i];
        cssDefaultStyles += property + ':' + getInputCSS(property) +';';
    }
    // End of CSS variable checks
    var text = input.value,
        textLen = text.length,
        fakeClone = document.createElement("div");
    if(selectionStart > 0) appendPart(0, selectionStart);
    var fakeRange = appendPart(selectionStart, selectionEnd);
    if(textLen > selectionEnd) appendPart(selectionEnd, textLen);
    // Styles to inherit the font styles of the element
    fakeClone.style.cssText = cssDefaultStyles;
    // Styles to position the text node at the desired position
    fakeClone.style.position = "absolute";
    fakeClone.style.top = topPos + "px";
    fakeClone.style.left = leftPos + "px";
    fakeClone.style.width = width + "px";
    fakeClone.style.height = height + "px";
    document.body.appendChild(fakeClone);
    var returnValue = fakeRange.getBoundingClientRect(); //Get rect
    if (!debug) fakeClone.parentNode.removeChild(fakeClone); //Remove temp
    return returnValue;
    // Local functions for readability of the previous code
    function appendPart(start, end){
        var span = document.createElement("span");
        span.style.cssText = cssDefaultStyles; //Force styles to prevent unexpected results
        span.textContent = text.substring(start, end);
        fakeClone.appendChild(span);
        return span;
    }
    // Computing offset position
    function getInputOffset(){
        var body = document.body,
            win = document.defaultView,
            docElem = document.documentElement,
            box = document.createElement('div');
        box.style.paddingLeft = box.style.width = "1px";
        body.appendChild(box);
        var isBoxModel = box.offsetWidth == 2;
        body.removeChild(box);
        box = input.getBoundingClientRect();
        var clientTop  = docElem.clientTop  || body.clientTop  || 0,
            clientLeft = docElem.clientLeft || body.clientLeft || 0,
            scrollTop  = win.pageYOffset || isBoxModel && docElem.scrollTop  || body.scrollTop,
            scrollLeft = win.pageXOffset || isBoxModel && docElem.scrollLeft || body.scrollLeft;
        return {
            top : box.top  + scrollTop  - clientTop,
            left: box.left + scrollLeft - clientLeft};
    }
    function getInputCSS(prop, isnumber){
        var val = document.defaultView.getComputedStyle(input, null).getPropertyValue(prop);
        return isnumber ? parseFloat(val) : val;
    }
}