I want to do something like this:

Open file:///C:/stats/analysis.html in the browser
C:\stats\analysis.html includes ./analysis.js
C:\stats\analysis.js loads each file in ./csvs/ and adds the number of lines to a tally (stupid but easy example)
C:\stats\csvs\ is a folder containing several files to be analysed

From what I gleaned from other questions here, it seems that require.js would make that possible if the files to be read were JSON files, but I have not found mention of how to do this for regular text files without requiring the user to "upload" the files through the <input type="file"> dialogue.

I'm choosing locally stored browser JS because the browser is already there to interpret the language (no additional installation of an interpreter needed), it works without internet (because it's locally stored), and you can look at the source code whenever you want (unlike the binaries of compiled languages).