You an set slider/dropdown value with slider1 = value; where 1 is the index of the slider.
This means that sliders can be used to display infos, or to be populate by other data, but they have to be declared first.
You can so also populate from a file, as long as you can transpose at one point or another these values to integer or float numbers. You can say "Populate my dropdown with "My String" ", but you can declare a drodown which have an index for "My String" and have value in your file to make it switch to "My String" index.
https://www.reaper.fm/sdk/js/js.php#js_file