Don't submit form after calling Javascript function

Don't submit form after calling Javascript function

I have this problem:
my HTML code:
<form name="addUser" method="post" action="../src/process_register.php">
....
<button type="submit"
onClick="formhash_register(this.form);">Submit</button>
</form>
my JavaScript code:
function formhash_register(form)
{
var username = form.inputUsername.value;
if(username == "")
return false;
else
form.submit();
}
Although my function returns false, you will go to another page, and thats
not what I wanted. Is there way to cancel submit process when the submit
button is pressed ?