How to check data inside field with a form and without jquery

Hello,

I am looking something allowing to check the number of character inside a textarea without jquery. There are some solution, but I like the code below because he is very short.

It's for some people want to do create a form as contact, feedback, reviews, comment ... It can be useful

If the number of character is not reach, a pop up appear on the screen.

  <form action="mango.php" method="post" id="form12" onsubmit="var text = document.getElementById('checkField').value; if(text.length < 80) { alert('put more info!'); return false; } return true;">
<textarea rows="10" cols="80" maxlength="200" required id="checkField" > </textarea>
</form>