requiring form input

I have a form with 20 check boxes and two text input boxes. One for email, the other for zip code. I would like to make it so that the email is required and if it isn't entered and the submit button is pushed a error window will pop up stating that their email address is required to proceed. How can I do that?

Thanks
timshankAsked:
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MHQConnect With a Mentor Commented:
Sorry, that was a slight misstake by me. Try using this code instead:
It should work just as you want it to.

<HTML>
<HEAD>
<SCRIPT>
function submit_page(form) {
            check = true;
            // CHECKES IF THERE IS AN ADRESS
        if(form.theField.value.length < 1) {
            alert("You left the Email field blank.");
            check = false;
        }

        // CHECKS IF IT IS A VALID ADRESS
            if((check) && form.theField.value.indexOf('@', 0) == -1) {
              alert("You did not enter a valid email address.");
                  check = false;
        }
            if (check)
                  form.submit();
    }
</SCRIPT>
</HEAD>

<BODY>
<form action="to your next page" name="theform">
<input type="text" name="theField">
<input type="button" value="SUBMIT" onClick="submit_page(this.form);">
</form>
</BODY>
</HTML>

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MHQCommented:
This code should do the trick:

<HTML>
<HEAD>
<SCRIPT>
function submit_page(form) {

            // CHECKES IF THERE IS AN ADRESS
        if(form.theField.value == "") {
            alert("You left the Email field blank.");
            return false;
        }

        // CHECKS IF IT IS A VALID ADRESS
            if(form.theField.value.indexOf('@', 0) == -1) {
              alert("You did not enter a valid email address.");
                  return false;
        }
            form.submit();      
    }
</SCRIPT>
</HEAD>

<BODY>
<form action="thepagetosendto.html" name="theform">
<input type="text" name="theField">
<input type="button" value="SUBMIT" onClick="submit_page(this.form)">
</form>
</BODY>
</HTML>
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timshankAuthor Commented:
The error window comes up, however after I click on the ok button it takes me away from the form page. How can I keep my visitors at the form page until they put their email address in?

Thanks
0
 
MHQCommented:
MHQ changed the proposed answer to a comment
0
 
MHQCommented:
I'd appreciate if you could take time to evaluate my answer and if it's to your liking, award me the points.
0
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