How can I submit a form (with inputs) with Javascript

Question

I'm working on an Asp.Net app and I have a form like below;

<form action="tcSubmit.asp" method="POST" name="mainForm" onsubmit="return (checkForm());">

// there are a lot of inputs here

</form>

Because of a few things I did in the checkForm method, I need to submit the form in another method that I call from checkForm.

I can submit the form as below, but it doesn't submit with the body.

document.forms['mainForm'].submit();

How can I add the body of the form to the request while submitting?

Can anyone help me? Thanks