A Perplexing Javascript Parsing Puzzle

It prints 1.

wait wtf

At the beginning of a line (and only at the beginning of a line), --> starts a comment. The JavaScript is parsed as

x = 1;
x; // 0 

The browser then displays the value of the last expression, which of course is 1.

but why

It’s a legacy hack.

Netscape Navigator 2 introduced both JavaScript and the <script> tag. Older browsers in common use (like Navigator 1) had no idea that <script> content was anything special and wrote it as regular text on the page. To ensure graceful degradation, webdevs would wrap their scripts in html comment blocks:

<script>
<!--
console.log("hello world!")
-->
</script>

Old browsers would parse the content as an HTML comment and ignore it, new browsers would parse the content as JavaScript and execute it. I’m not quite sure why <!-- and --> weren’t syntax errors; presumably there was special code in the js engines to handle them, but I can’t figure it out where.

All modern browsers at least recognize <script>.¹ But since some old websites still have the hack and the standardization committee will never, ever break the web, they added <!-- and --> as legal comment tokens to the 2015 standard.

<!-- and --> both act like //, i.e. starting line comments. --> is only valid at the start of a line (to avoid ambiguity with a postfix decrement followed by a greater than operator), while <!-- can occur anywhere in the line. — MDN Web Docs

Web browsers are required to support this syntax, while other engines are not. Node and Electron both support it, though, as they share Chromium’s v8 engine.