The Beacon API was explicitly designed to allow the asynchronous queuing of network requests when navigating away from pages. While this event is supported in iOS Safari, you haven’t been able to make any network requests in an attached event handler in Safari version 11.1 up to and including 12.1. Browser support is as good if not better than both the beforeunload and unload events.
The event is issued when people navigate away from your page even when it’s not being unloaded and it doesn’t have the performance penalty of the other two events. So, what is a web developer to do instead? Use the pagehide event instead. Safari on MacOS can use the beforeunload event but I’d discourage it because of its performance impact. Because these events don’t work on iOS Safari, people misunderstand what’s going on and assume that the Beacon API is to blame when they fail to queue a request in event handlers attached to these events. These are standard events and part of the web platform, but WebKit has chosen not to emit them. These events are also unsupported in iOS Safari. This in turns means the pages can’t be cached and quickly restored when visitors move back and forth in their browser history or restore a previous browsing session. These two events are fired as pages are being unloaded, and notably attaching event listeners to these events causes webpages to be unloaded when navigating away from them. Many examples of the Beacon API execute it in a handler listening to either the beforeunload or unload events. It’s being used for all sorts of things, but its main benefit over other network request methods is that it allows for queuing a submission as the visitor leaves the page or the page is being unloaded. The Beacon API is a web platform standard that was designed to allow webpages to send some data in an asynchronous POST request without causing a performance penalty for the visitor. The Beacon API is available in all current mainstream web browsers, and was introduced in Safari in version 11.11 (iOS 11.3 and MacOS 10.13.) However, Safari have had some trouble delivering on the promises of this web API especially on iOS.
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