Converting variable to form variable

I am an amateur JS coder and have been unable to find a solution to this problem on the web...

I have a mixture of form variables and javascript variables that I need to mix together into one set of form variables to be posted.

Does anyone know how I can combine literal form elements with javascript variables (somehow converted into form variables at the time of the post) into one post?

Thanks-

If you have a pre-set number of things, use hidden form fields and set the values before submit.
If a dynamic number of things, dynamically create new form fields and set them before submit.

Of course, either of these require script on the client, and so should have a fallback for users with no script on sites that must meet ADA requirements.