Same Script, Multiple Tags
Today, I happened on a fun quirk of web-dev. What happens if you include the same script twice, say like this:
<!DOCTYPE html>
<html lang="en">
<head>
<script src="test.js" type="module"></script>
<script src="test.js" type="module"></script>
test.js only runs once, despite being included twice. It seems modules have a specific trait where they're only ever evaluated once, I believe specified in steps 4 and 5 of ECMAScript 6.0 section 15.2.1.16.5.
Nothing good ever comes of running scripts multiple times. I'm glad it's out.
P.S.: Fun fact: You can still document.write(…) in an ECMAScript module, which just feels… wrong. (At least the with statement is history there, as modules are always in strict mode.)