onSubmit return false not working - submit form by Javascript

I need to submit the form by using Javascript, but it seems the functions in onSubmit event don't work. When I click on Send link, the form is submitted, but I will not stay on the same page. It will redirect me to web page included in action attribute. Could you please tell me why is it not working ?

<form id="idmessage" name="message" method="post" onSubmit="javascript: alert('Form submitted'); return false;" action="index.php" >
<textarea name="content" ><textarea>
<br />
<a href="javascript:void(0);" onClick="javascript: document.getElementById('idmessage').submit(); " >Send</a>
</form>

Hi,

The functions included in onSubmit event don't work, when you are using javascript function .submit() to submit the form. You have to call required javascript functions before document.getElementById('idmessage').submit();

For example:

<form id="idmessage" name="message" method="post" action="index.php" >
<textarea name="content" ><textarea>
<br />
<a href="javascript:void(0);" onClick="javascript: var valid = formValidation(); if(valid=='yes'){ alert('Form submitted'); document.getElementById('idmessage').submit(); }else { alert('Form not submitted'); } " >Send</a>
</form>