js实现输入框焦点中插入文字

以下是参考网上的资料，具体的网址现在没有找到，实现的功能类似于微博的发表窗口中在光标的焦点处插入文字

var focus_tag = { first:function(area){
          area.focus();
          if (area == null) {
              area = event.srcElement;
          }
          if(document.selection){
              var rtextRange = area.createTextRange();
              rtextRange.moveStart('character', area.value.length);
              rtextRange.collapse(true);
              rtextRange.select();
          }
    },getCursorPosition:function(textarea) {
            var rangeData = {text: "", start: 0, end: 0 };
            textarea.focus();
        if (textarea.setSelectionRange) { // W3C
            rangeData.start= textarea.selectionStart;
            rangeData.end = textarea.selectionEnd;
            rangeData.text = (rangeData.start != rangeData.end) ? textarea.value.substring(rangeData.start, rangeData.end): "";
        } else if (document.selection) { // IE
            var i,
                oS = document.selection.createRange(),
                oR = document.body.createTextRange();
            oR.moveToElementText(textarea);

            rangeData.text = oS.text;
            rangeData.bookmark = oS.getBookmark();
            for (i = 0; oR.compareEndPoints('StartToStart', oS) < 0 && oS.moveStart("character", -1) !== 0; i ++) {
                if (textarea.value.charAt(i) == '\n') {
                    i ++;
                }
            }
            rangeData.start = i;
            rangeData.end = rangeData.text.length + rangeData.start;
        }

        return rangeData;
    }, setCursorPosition:function(textarea, rangeData) {
        if(!rangeData) {
            alert("You must get cursor position first.");
        }
        if (textarea.setSelectionRange ) { // W3C
            textarea.focus();
            textarea.setSelectionRange(rangeData.start, rangeData.end);
        } else if (textarea.createTextRange) { // IE
            var oR = textarea.createTextRange();
            if(textarea.value.length === rangeData.start) {
                oR.collapse(false);
                oR.select();
            } else {
                oR.moveToBookmark(rangeData.bookmark);
                oR.select();
            }
        }
    },add:function (textarea, rangeData,text) {
        var oValue, nValue, oR, sR, nStart, nEnd, st;
        this.setCursorPosition(textarea, rangeData);
        if (textarea.setSelectionRange) { // W3C
            oValue = textarea.value;
            nValue = oValue.substring(0, rangeData.start) + text + oValue.substring(rangeData.end);
            nStart = nEnd = rangeData.start + text.length;
            st = textarea.scrollTop;
            textarea.value = nValue;
            if(textarea.scrollTop != st) {
                textarea.scrollTop = st;
            }
            textarea.setSelectionRange(nStart, nEnd);
        } else if (textarea.createTextRange) { // IE
            sR = document.selection.createRange();
            sR.text = text;
            sR.setEndPoint('StartToEnd', sR);
            sR.select();
        }
    }