Untitled

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>example</title>
  <link rel="stylesheet" href="style.css">

</head>
<body>
  <button id="xBtn" style="font-size:20px">テキストをJSで挿入</button><br>
  <p>
    普通に書き換え<br>
    <textarea id="xT1"></textarea>
  </p>
  <p>
    execCommand('insertText')<br>
    <textarea id="xT2"></textarea>
  </p>
  <p>
    execCommand('ms-beginUndoUnit')+<br>
    execCommand('ms-endUndoUnit')<br>
    <textarea id="xT3"></textarea>
  </p>
  <script src="script.js"></script>
</body>
</html>

// Code goes here

var byId = function(id){ 
  return document.getElementById(id);
};

var btn = byId('xBtn');
var t1 = byId('xT1');
var t2 = byId('xT2');
var t3 = byId('xT3');

btn.addEventListener('click',function(){
  var newText = 'hello\n';
  //普通に書き換え
  t1.value = t1.value += newText;
  //execCommand('insertText')
  t2.focus();
  document.execCommand('insertText', false, newText);
  //MS対応版
  try {
    document.execCommand('ms-beginUndoUnit');
  } catch (e) {}
  t3.value = t3.value += newText;
  try {
    document.execCommand('ms-endUndoUnit');
  } catch (e) {}
});

/* Styles go here */

textarea{
  min-height:80px;
}