Some people are getting a popup in iOS or macOS Safari to download a file named “f.txt.js”. If you are getting this popup, do not download this file. It is malware, here is what it does,
f.txt.js is a browser hijacker application that will generate ads and page-redirects from within your web browser, such as Safari. So long as you have this kind of browser hijacker on your system, you will constantly be barraged with pop-up ads, banner notifications, redirect links and other messages prompting you to click on them or to visit a certain website.
This article explains how to check for this malware and ways to remove it if it is on your system. It also provides preventative suggestions.