Which web language can read and write to local text files?

I know SO is going to ding me for this one, but I really need help! I have an html-based app and I'm just trying to read and write to a text file (ultimately to import data into Excel). Both the html and text file are on my local machine; I am the only user. I thought this would be simple Javascript, but apparently not. After ~2 hours of research, I'm more confused then when I started. I have these ideas of how it might be done, such as:

-maybe it actually is possible with simple javascript?

-maybe I need to host the files on a server and use something like PHP or AJAX?

-maybe I need some special libraries and use something like JSON or JQuery?

but I don't know which if any of those ideas will work.

Any advice is greatly appreciated. Thanks.