js获取光标位置例子

<html>
<head>
<title>TEST</title>
<style>
body,td {
font-family: verdana, arial, helvetica, sans-serif;
font-size: 12px;
}
</style>
<script type="text/javascript">
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("startId").value = start;
  document.getElementById("endId").value      = end;
}
</script>
</head>
<body>
<form action="a.cgi">
  <table border="1" cellspacing="0" cellpadding="0">
   <tr>
    <td>start: <input type="text" id="startId" size="3" /></td>
    <td>end:   <input type="text" id="endId" 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>
</body>
</html>