Here's my approach using Fetch API and async function
<div class="js-component" data-name="header" data-ext="html"></div>
<div class="js-component" data-name="footer" data-ext="html"></div>
<script>
const components = document.querySelectorAll('.js-component')
const loadComponent = async c => {
const { name, ext } = c.dataset
const response = await fetch(`${name}.${ext}`)
const html = await response.text()
c.innerHTML = html
}
[...components].forEach(loadComponent)
</script>