在JS中如何实现指定位置插入内容

function insertAtCursor(myField, myValue) { 
 
 //IE 浏览器 
 if (document.selection) { 
  myField.focus(); 
  sel = document.selection.createRange(); 
  sel.text = myValue; 
  sel.select(); 
 } 
 
 //FireFox、Chrome等 
 else if (myField.selectionStart || myField.selectionStart == '0') { 
  var startPos = myField.selectionStart; 
  var endPos = myField.selectionEnd; 
 
  // 保存滚动条 
  var restoreTop = myField.scrollTop; 
  myField.value = myField.value.substring(0, startPos) + myValue + myField.value.substring(endPos, myField.value.length); 
  
  if (restoreTop > 0) { 
  myField.scrollTop = restoreTop; 
  } 
  
  myField.focus(); 
  myField.selectionStart = startPos + myValue.length; 
  myField.selectionEnd = startPos + myValue.length; 
 } else { 
  myField.value += myValue; 
  myField.focus(); 
 } 
} 
<textarea id="textarea" style="width: 386px; height: 260px"> 
</textarea> 
<input type="text" id="text" /> 
<input type="button" value="插入" onclick="insertAtCursor(document.getElementById('textarea'),document.getElementById('text').value)" />