Is there a way to know when a download is started??

My backend have to download data from several sources and build a zip file in realtime.
It takes a few seconds to prepare the download, so when the user clicks the button to save it, it's very easy for users to spam the button and trigger several downloads.

I know I can just disable the button for a few seconds, but it's a guessing solution.

If an event triggered when the download actually starts I could keep the button disabled for the correct amount of time.