execCommand.js

var copyBtn = document.querySelector('#copy');
copyBtn.addEventListener('click', function (event) {
  event.preventDefault();

  try {
    document.querySelector('#code').select();
    document.execCommand('copy');
  }
  catch (error) {
    alert('Sorry, leider konnte nicht kopiert werden.');
  }
});

form.html

<form class="form-horizontal">
<div class="form-group">
  <label for="code" class="col-sm-2 control-label">Code</label>

  <div class="col-sm-10">
<textarea name="code" id="code" cols="90" rows="5">var code = 'Dieser Code muss kopiert werden.';
console.log(code);</textarea>
  </div>
</div>
<div class="form-group">
  <div class="col-sm-offset-2 col-sm-10">
    <button id="cut" type="submit" class="btn btn-default">Cut</button>
    <button id="copy" type="submit" class="btn btn-default">Copy</button>
  </div>
</div>
</form>