Javascript won't read the contents of the folder. You will have to make up a javascript array with the file names as a .js file and have javascript reference it.
Is there a way to populate a drop down list with filenames in a particular folder ?
Yes.
Now would you like to ask a less abstract question, perhaps supplying some useful information like the form in which your list currently exists, and in what order the filenames are to be arranged?
@jmrker You are correct. Not all servers use php, but 77.9% of them do. IMHO that means its a good assumption. It was not too long ago when I'd be told that I should not recommend coding in javascript because "not all people have it turned on in their browsers" At that time 90% of the users did have it turned on.
I also assumed that the folders could be on the server. I don't think js can read those by it's self. While php would read a file server side or user side. So I still think it's the better choice. Maybe I should have said server side language instead of php, but I only know php and can not say the other can do the job.