Tag in comment

importance: 3

What does this code show?

<script>
 let body = document.body;
 body.innerHTML = "<!--" + body.tagName + "-->";
 alert( body.firstChild.data ); // what's here?
</script>

The answer: BODY.

<script>
 let body = document.body;
 body.innerHTML = "<!--" + body.tagName + "-->";
 alert( body.firstChild.data ); // BODY
</script>

What’s going on step by step:

The content of <body> is replaced with the comment. The comment is , because body.tagName == "BODY". As we remember, tagName is always uppercase in HTML.
The comment is now the only child node, so we get it in body.firstChild.
The data property of the comment is its contents (inside ): "BODY".