HTML comments are allowed in Javascript, for older browsers which are not aware of <script> tags.
Ref: Are HTML comments inside script tags a best practice?
So, the JS parser allows <!-- at the start of script, which will be ignored for parsing. And, --> should be prefixed with // so that JS engine ignores them too.
Ref W3C: https://www.w3.org/TR/REC-html40/interact/scripts.html#h-18.3.2
While playing with this, I saw this weird behaviour.
<script>
<!--
alert(1);
-->
</script>
Here, note that --> isn't prefixed by //, which should throw an error by Chrome JS engine. But, Chrome JS engine consider them to be comments, and ignore parsing. Why? (Q1)
And, another behaviour:
<script>
alert(1);
-->''>{][}
</script>
This snippet will execute alert. Here, note the place of -->. This specific sequence of characters makes the parser ignore all other incorrect values following it.
What I mean is, in place of -->, use any other character/sequence, it will throw error.
So, what's happening here? How --> makes this snippet valid? (Q2)
Note: --> sequence should be prefixed with a new line. That is,
<script>
<!--
alert(1); -->''>{][}
</script>
will throw an error. Again, why? (Q3)
