js实现输入框焦点中插入文字

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(); 
        } 
    } 
}