How does javaScript affect the auto save feature?

Hi, I've noticed that when we have a field that's being populated via javaScript, the auto save isn't always working. For instance, if we leave the form, the fields involved the javaScript calculation don't save. I know I'm being a bit vague, but hopefully others have experienced what's going on. 

Is there any good documentation around this? Right now, it's been hard to understand exactly when and why it's happening. Having a better understanding of when we cannot just leave a form without auto save kicking in would be great. 

On this note, is it possible to write javaScript or a plugin that will automatically save a form whenever the user leaves that form?