JSFiddle - React, Tailwind, and code Playground

HTML

<div id="app">
  <button>2333</button>
  <br>
  <div id="result"></div>
</div>

JavaScript

function haha() {
  console.log('haha')
  const result = document.querySelector('#result')
  
  result.insertAdjacentHTML('beforebegin',  '<iframe id="iframe"></iframe>')
  
  const el = document.querySelector('#iframe')
  const doc = el.contentDocument
  
  doc.body.innerHTML = 'Something'
  
  setTimeout(() => {
    result.innerHTML = [
      el === document.querySelector('#iframe'),
      doc === document.querySelector('#iframe').contentDocument, 
    ].join(',')
  }, 0)
}

const btn = document.querySelector('button')
btn.addEventListener('click', haha)