var start=0; 

var end=0; 

function add(){ 

var textBox = document.getElementById("ta"); 

var pre = textBox.value.substr(0, start); 

var post = textBox.value.substr(end); 

textBox.value = pre + document.getElementById("inputtext").value + post; 

} 

function savePos(textBox){ 

//如果是Firefox(1.5)的话，方法很简单 

if(typeof(textBox.selectionStart) == "number"){ 

start = textBox.selectionStart; 

end = textBox.selectionEnd; 

} 

//下面是IE(6.0)的方法，麻烦得很，还要计算上'\n' 

else if(document.selection){ 

var range = document.selection.createRange(); 

if(range.parentElement().id == textBox.id){ 

// create a selection of the whole textarea 

var range_all = document.body.createTextRange(); 

range_all.moveToElementText(textBox); 

//两个range，一个是已经选择的text(range)，一个是整个textarea(range_all) 

//range_all.compareEndPoints()比较两个端点，如果range_all比range更往左(further to the left)，则 //返回小于0的值，则range_all往右移一点，直到两个range的start相同。 

// calculate selection start point by moving beginning of range_all to beginning of range 

for (start=0; range_all.compareEndPoints("StartToStart", range) < 0; start++) 

range_all.moveStart('character', 1); 

// get number of line breaks from textarea start to selection start and add them to start 

// 计算一下\n 

for (var i = 0; i <= start; i ++){ 

if (textBox.value.charAt(i) == '\n') 

start++; 

} 

// create a selection of the whole textarea 

var range_all = document.body.createTextRange(); 

range_all.moveToElementText(textBox); 

// calculate selection end point by moving beginning of range_all to end of range 

for (end = 0; range_all.compareEndPoints('StartToEnd', range) < 0; end ++) 

range_all.moveStart('character', 1); 

// get number of line breaks from textarea start to selection end and add them to end 

for (var i = 0; i <= end; i ++){ 

if (textBox.value.charAt(i) == '\n') 

end ++; 

} 

} 

} 

document.getElementById("start").value = start; 

document.getElementById("end").value = end; 

} 
<form action="a.cgi"> 

<table border="1" cellspacing="0" cellpadding="0"> 

<tr> 

<td>start: <input type="text" id="start" size="3"/></td> 

<td>end: <input type="text" id="end" size="3"/></td> 

</tr> 

<tr> 

<td colspan="2"> 

<textarea id="ta" onKeydown="savePos(this)" onKeyup="savePos(this)" onmousedown="savePos(this)" onmouseup="savePos(this)" 

onfocus="savePos(this)" rows="14" cols="50"></textarea> 

</td> 

</tr> 

<tr> 

<td><input type="text" id="inputtext" /></td> 

<td><input type="button" onClick="add()" value="Add Text"/></td> 

</tr> 

</table> 

</form>