如何在body标签中插入insertBefore（）元素？

var p = document.createElement(\"p\");
p.innerHTML = \"test1\";
document.body.insertBefore(p, null);

var p = document.createElement(\"p\");
p.innerHTML = \"test2\";
document.body.insertBefore(p, null);

document.body.insertBefore(p, document.getElementsByTagName(\'body\')[0]);

firstChild

body

const p = document.createElement(\"p\");
p.textContent = \"test1\";
document.body.insertBefore(p, document.body.firstChild);

document.body.prepend(p);

document.body.prepend(\'This text!\');

document.getElementsByTagName(\'body\')[0]

var body   = document.body || document.getElementsByTagName(\'body\')[0],
    newpar = document.createElement(\'p\');
newpar.innerHTML = \'Man, someone just created me!\';
body.insertBefore(newpar,body.childNodes[0]);

document.body

document.documentElement

body