SaveStreams is the solution to saving streams in the web browser. It is perfect for web apps where there's a need to save large amounts of data on devices with e.g. limited RAM.

SaveStreams takes a different approach. Instead of saving data in client-side storage or in memory you could now actually create a writable stream directly to the file system (I'm not talking about chromes sandboxed file system or any other web storage). This is accomplish by emulating how a server would instruct the browser to save a file using some response header + service worker

If the file you are trying to save comes from the cloud/server use the server instead of emulating what the browser does to save files on the disk using SaveStreams. Add those extra Response headers and don't use AJAX to get it.

If the file you are trying to save comes from the cloud/server use the server instead of emulating what the browser does to save files on the disk using SaveStreams. Add those extra Response headers and don't use AJAX to get it. If you can't change the headers then you may use StreamSaver as a last resort. saveStreams and others alike are mostly for client generated content inside the browser.

Best practice

Use https if you can. That way you don't have to open the man in the middle in a popup to install the service worker from another secure context. Popups are often blocked but if you can't it's best that you initiate the createWriteStream on user interaction. Even if you don't have any data ready - this is so that you can get around the popup blockers. (In secure context this don't matter) Another benefit of using https is that the mitm-iframe can ping the service worker to prevent it from going idle. (worker goes idle after 30 sec in firefox, 5 minutes in blink) but also this won't mater if the browser supports transferable streams throught postMessage since service worker don't have to handle any logic. (the stream that you transfer to the service worker will be the stream we respond with)

Handle unload event when user leaves the page. The download gets broken when you leave the page. Because it looks like a regular native download process some might think that it's okey to leave the page beforehand since it's is downloading in the background directly from some a server, but it isn't.