No offense, but this has jack **** to do with JSON.stringify. This is 100% the fault of the server side developer blindly trusting that client-side data EXISTS, and whoever wrote the HTML likely not building the form properly!

That said, why the JSON crap if this is coming from a FORM? That's the formData object's job! The numeric form input numbers likely doing nobody any good either...

This is why so much of what people do with JS is overcomplicated convoluted rubbish. Hard coding junk to avoid having proper HTML and leveraging existing functionality.

This is not JSON.stringify's fault. This is a comedy of errors likely stemming from a poorly coded HTML form, getting JSON involved whatsoever for no good reason, and server-side developer incompetence blindly trusting client-side data.

the numeric "fieldID" crap ALONE being giant warning signs that the HTML is likely utter and total drek.