Download file - what is window.requestFileSystem()

I struggle a lot with downloading a file in the way that the file is visible to a user of a mobile device.
In many examples people use something like that:
window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function(directory)
for example, here:

but i don’t understand that because here:

is written “The non-standard Window method requestFileSystem() method is a Google Chrome-specific method which lets a web site or app gain access to a sandboxed file system for its own use.” and " It has even been removed from the proposed specification. Do not use this method!" so what is going on? How am I suppose to download a file and make it visible to a user? I am start thinking such a trivial task is not possible with ionic.
Here is my stackoverflow topic: