Checking if text area is blank prior to form submission? - forms

Question

So I wrote this code so that every time someone clicks the submit button, a javascript
function, called check, will see if the text area is empty. If it is not, then the form will be submitted.
This is my code. Why isn't it working?
<form method=post name='form_post' id='form_post' action='SUBMITIT.PHP'>
<textarea id=message name=message class=post onfocus=this.className='post_focus';
placeholder='Share your thoughts' maxlength=500></textarea>
<br>
<button onclick='check()' id=button name=button>Post</button>
</form>
<script type="text/javascript">
function check()
{
if(!document.textArea.message.value=="")
{
document.forms["form_post"].submit();
}
}
</script>
Thanks!
EDIT: I finally got it to work. Here's a template if you are having a similar problem.
<!--The form-->
<form action="mypage.php" method="post" name="myForm" id="myForm">
<textarea name=myTextArea id=myTextArea>
</textarea>
</form>
<br>
<button onclick='check()'>
Post
</button>
<!--The script that checks-->
<script type="text/javascript">
function check(){
String.prototype.trim = function() { return this.replace(/^\s+|\s+$/g, ""); };
var textAreaValue=document.getElementById('myTextArea').value;
var trimmedTextAreaValue=textAreaValue.trim();
if(trimmedTextAreaValue!="") {
document.forms["myForm"].submit();
}
}
</script>

Answer 1

The following woks, and it also wipes unnecessary spaces, the form will only submit when given a character
<form action="yourpage.php" method="post" onsubmit="return check(this)">
<textarea id="message"></textarea>
<input type="submit" value="Post" />
</form>
<script>
function check(form){
if (form.message.value.trim() == ""){
return false
}
}
</script>
This is the most simple way to do this, and the advised one.

Answer 2

You could simply add "required" to the textarea field in HTML.
Note: Although the question is quite old, I am adding the answer for the future references.