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>

Update September 2020: life's almost good. Edge Chromium is widely rolled out, and Safari 14.0 ships soon with a fix.

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	✅
			11-13.1	v		module	✅⚠
79+	60+	61+	14.0+	v		module	✅

Summary:

• ✅ no browser does double execution (provided the Safari hack)
• ✅ modern Chrome, Edge, 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)
  • This will apparently be fixed in Safari 14.0 (October 2020)
• ❌ pre-2018 browsers do double fetches
• ❌❌ Edge 17-18 does triple fetch (2x module + 1x nomodule). This about 80% of all Edge as of April 2020, but being phased out by Edge 80+.

@indexzero I think I made PR to caniuse before I found out Safari 11+ double fetching, not sure. My PR touches only Edge.

Agree there's inconsistency - Edge 16-18 and Safari 10.1 should be marked with same color on caniuse (whether or yellow or green, I can't tell; not sure what is the meaning; probably yellow up to Edge 18 and Safari 13/ ios 13.2 inclusive, due to double fetching?)

It appears this didn't make it into Safari 13.1 😢

@jakub-g, I think normal loading works on the new Chromium-based Edge now. I ran your page, and I think it only downloads module.js, but I wasn't sure if you were testing other edge cases too.

Could you please, @AndrewGibson27, provide a link with where you saw that? Thanks!

It isn't working for me in Safari 13.1 either:

It does work in Safari Technology Preview 104 which corresponds a the future Safari 13.2 release.

Safari 13.1 is based on Webkit TP98 [0]

https://bugs.webkit.org/show_bug.cgi?id=194337 was closed on 23 Jan 2020 which means it was not in TP99 (released 22 Jan 2020), it was probably shipped in TP100 (05 Feb 2020).

The bug fix is hence expected in Safari 14.0 in October 2020.

[0] https://gist.github.com/jakub-g/48a16195280a7023f570ffa5c8a4eae5
[1] https://webkit.org/blog/9992/release-notes-for-safari-technology-preview-99/
[2] https://webkit.org/blog/10024/release-notes-for-safari-technology-preview-100/