NTOONT,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
document.addEventListener( "DOMContentLoaded" , function() {
const scriptText = `
for(let i=1; i<=3; i++) {
alert("Из шляпы достали "+i+" кролика!")
}
`;
function setHeader() {
const header = document.createElement("div");
const script = document.createElement("script");
script.textContent = scriptText;
header.append(script);
header.append('dfbdfb');
document.body.insertAdjacentElement('afterbegin', header);
}
setHeader();
});
</script>
</head>
<body>
</body>
</html>