How can I update a Joomla form editor box contents from Javascript?

Question

I have a form with lots of fields including editor fields that display using jce/codemirror. I have buttons that call the submitbutton.js which builds a string from other elements in the form. I can use a function to update the text field by using

document.getElementById('jform_title').value = com_title.concat(', ', com_version, ', ', com_exploit);

but I cannot do the same to an editor window due to the way Joomla renders it. Any attempt to access the elements results in javascript saying Uncaught TypeError.

I read that

Joomla.editors.instances['jform_description'].setValue(com_descript)

would work but it doesn't update the form.

Any thoughts?