app.js

document.querySelector(".bold").addEventListener('click', addBold, false);

ta = document.querySelector("textarea[name='content']");

function addBold() {
	surroundInTextarea(ta, '**');
  return false;
}

function typeInTextarea(el, newText) {
  var start = el.selectionStart
  var end = el.selectionEnd
  var text = el.value
  var before = text.substring(0, start)
  var after  = text.substring(end, text.length)
  
  el.value = (before + newText + after)
  el.selectionStart = el.selectionEnd = start + newText.length
  el.focus()
}

function surroundInTextarea(el, newText) {
  var start = el.selectionStart
  var end = el.selectionEnd
  var text = el.value
  var before = text.substring(0, start)
  var selection = text.substring(start, end)
  var after  = text.substring(end, text.length)
  
  el.value = (before + newText + selection + newText + after)
  el.selectionStart = start + newText.length
  el.selectionEnd = start + newText.length + selection.length
  
  el.focus()
}

index.html

<button class="bold">B</button>
<textarea name="content"></textarea>