upload/download files client-side only

sample.html

<input type="file" onchange="fileSelected(this)" accept="application/JSON" />
<a
   download="settings.json"
   target="_blank"
>download</a>
<pre>
	<code></code>
</pre>

sample.js

const preview  = document.querySelector('code');
const download = document.querySelector('a');

function fileSelected(ele) {
  const file = ele.files[0];
  const fr   = new FileReader();

  fr.onload = () => {
    preview.innerText = fr.result;
    download.href     = window.URL.createObjectURL(file);
  };

  fr.readAsText(file);
}