摘自 https://blog.csdn.net/gisredevelopment/article/details/29255147
纯记录，没啥可说的
<html>
<head>
<script type='text/javascript'>
function test(str){
var tc = document.getElementById("mytextarea");
var tclen = tc.value.length;
tc.focus();
if(typeof document.selection != "undefined")
{
document.selection.createRange().text = str;
}
else
{
tc.value =
tc.value.substr(0, tc.selectionStart) +
str +
tc.value.substring(tc.selectionStart, tclen);
}
}
</script>
</head>
<body>
<textarea rows=5 name=s1 cols=27 id="mytextarea">aabb</textarea>
<input type=button onclick="test('这是需要加入的文字')" />
</body>
</html>