Will it double-fetch? Browser behavior with `module` / `nomodule` scripts

double-fetch-triple-fetch.md

Testing double-fetching of module/nomodule JS code (including the Safari hack)

<script type="module" src="module.js"></script>
<script nomodule src="nomodule.js"></script>

Test page: https://jg-testpage.github.io/es-modules/module-nomodule/

IE/Edge	Firefox	Chrome	Safari	fetches module	fetches nomodule	executes
15-	59-	55-	10.0-	v	v	nomodule	❌
16			10.1/3	v	v	module	❌
17-18				double!	v	module	❌❌
		56-60			v	nomodule	✅
	60+	61+	11.0+*	v		module	✅

Summary:

• ✅ no browser does double execution (provided the Safari hack)
• ✅ modern Chrome and Firefox never fetch more than necessary
• ⚠ Safari <11 may or may not double fetch (even with the hack); it does not on small test pages, but in real complex pages it does (it seems deterministic, but not clear what's the exact trigger)
• ⚠ Safari 11+ may still double fetch in some cases (see https://bugs.webkit.org/show_bug.cgi?id=194337)
• ❌ pre-2018 browsers do double fetches
• ❌❌ latest Edge does triple fetch (2x module + 1x nomodule)