Problem with Javascript form submission

Question

Would someone correct the following the Javascript function?
It should submit the form when test==true, and should give an alert message
when test==false.
The problem is when the alert message is displayed and closed, the form is still submitted.
In fact, I need no action when an alert message is closed.
Thank you.
<script type='text/javascript'>
function submitForm(en){
var test =true;//true means form is valid and ready for submission.
if(en == "submit_btn"){
var alertMessage =null;
if(myform.choice01.selectedIndex==0 && myform.choice02.selectedIndex==0 ){
     test=false;
     alertMessage ="ERROR1";
if(myform.category.selectedIndex==0){test = false; alertMessage="ERROR2";}
if(myform.left_right.selectedIndex==0){test = false; alertMessage="ERROR3";}
if(test==false) {alert(alertMessage); return true;}//dont submit
else if(test==true){//submit form
     var submitURL= "someURL";
     myform.action = submitURL;
     myform.submit();
          }//end else if
     }//end function
</script>

Answer 1

Try This
<FORM name="myform" method="POST" >
<SELECT name="category" onchange="submitForm(this.name)";>
//<Option> elements
<SELECT name="left_right" onchange="submitForm(this.name)";>
//<Option> elements
// and more <SELECT> elements
<INPUT type=submit name=submit_btn value="submit" onclick="submitForm(this.name)";>
</form>
<script type='text/javascript'>
function submitForm(en)
     myform = document.forms(0);
     var isValidRequest = true;//true means form is valid and ready for submission.
     if(en == "submit_btn")
          var alertMessage =null;
          if(myform.choice01.selectedIndex==0 && myform.choice02.selectedIndex==0 )
               isValidRequest=false;
               alertMessage ="ERROR1";
          if(myform.category.selectedIndex==0)
               isValidRequest = false;
               alertMessage="ERROR2";
          if(myform.left_right.selectedIndex==0)
               isValidRequest = false;
               alertMessage="ERROR3";
     if(isValidRequest)
          //submit form
          var submitURL= "someURL";
          myform.action = submitURL;
          myform.submit();
     }//dont submit
     else
          alert(alertMessage);
     }//end else
}//end function
</script>