jakub-g/double-fetch-triple-fetch.md

## double-fetch-triple-fetch.md

      
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    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+.
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	✅