index-post.html

<input type="file" id="selector" multiple>
<button onclick="upload()">Upload</button>

<div id="status">No uploads</div>

<script src="//code.jquery.com/jquery-3.1.0.min.js"></script>
<script type="text/javascript">

 function upload() {
   [$('#selector')[0].files].forEach(fileObj => {
     var file = fileObj[0]
     // Retrieve a URL from our server.
     retrieveNewURL(file, obj => {
       // Upload the file to the server.
       uploadFile(file, obj)
     })
   })
 }

 // Request to our Node.js server for an upload URL.
 function retrieveNewURL(file, cb) {
   $.get(`/presignedUrl?name=${file.name}`, (obj) => {
     cb(obj)
   })
 }

 // Use XMLHttpRequest to upload the file to S3.
 function uploadFile(file, obj) {
     var xhr = new XMLHttpRequest ()
     var objSon = JSON.parse(obj);
     xhr.open('POST', objSon.urlStr.postURL, true);

     var fd = new FormData();
     Object.entries(objSon.urlStr.formData).forEach(([key, value]) => {
         fd.append(key, value);
     });

     fd.append('file', file);
     xhr.send(fd);

     xhr.onload = () => {
       if (xhr.status == 204) {
         $('#status').text(`Uploaded ${file.name}.`)
       }
     }
 }

</script>