Tag in comment

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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".