Reducing resistor in the filament circuit (20th century retro script)

In accordance with the declaration that I am reproducing the content of the Fonar website, I am posting in this section, without any changes or modifications, a script that allows the calculation of the parameters of a drop-out resistor in a vacuum tube filament circuit. This script was created by modifying an applet I wrote in the 1990s and added to the Fonar website in the early 21st century. From a modern JavaScript perspective, it contains many unused elements. From a practical perspective, the script's advantages are limited, as we currently have programs for simulating electronic circuits. It can be used by anyone who wants to perform quick calculations without even implementing a simple circuit into the program. The script may also be interesting for someone who programs in JavaScript and would like to see what a script written a quarter of a century ago looks like.

I should note in advance that due to the program's age, it may have problems running in some browsers. I don't experience any. I invite you to view, try, and analyze the source code:

A reducing resistor in the filament circuit of an electron tube