let databox;
function initiate() {
  databox = document.getElementById("databox");
  const myfiles = document.getElementById("myfiles");
  myfiles.addEventListener("change", process);
}
function process(event) {
  databox.innerHTML = "";
  const files = event.target.files;
  const myfile = files[0];
  const reader = new FileReader();
  reader.addEventListener("load", function(event) {
    show(event, myfile);
  });
  const blob = myfile.slice(0, 1000);
  reader.readAsArrayBuffer(blob);
}
function show(event, myfile) {
  const buffer = event.target.result;
  databox.innerHTML = "Name: " + myfile.name + "<br>";
  databox.innerHTML += "Type: " + myfile.type + "<br>";
  databox.innerHTML += "Size: " + myfile.size + " bytes<br>";
  databox.innerHTML += "Blob size: " + buffer.byteLength + " bytes<br>";
}
window.addEventListener("DOMContentLoaded", initiate);