Jquery wrapText Fucntion in Coffeescript

gistfile1.coffee

wrapText = (elementID, openTag, closeTag) ->
  textArea = $("##{elementID}")
  len = textArea.val().length
  start = textArea[0].selectionStart()
  end = textArea[0].selectionEnd()
  selectedText = textArea.val().substring(start, end)
  replacement = "#{openTag}#{selectedText}#{closeTag}"
  textArea.val(textArea.val().substring(0, start) + replacement + textArea.val().substring(end, len))

#example click function
$('#bold_button').click (event) ->
  wrapText('post_body', '[b]', '[/b]')
  event.preventDefault()