JSFiddle - React, Tailwind, and code Playground

HTML

<input id="myInput" type="file">    
<button id="myBtn">Try it</button>
<span id="file"></span>

JavaScript

document.getElementById("myBtn").addEventListener("click", function() {
  uploadFile1();
}); 


function uploadFile1(){
  var files = myInput.files[0];
  var reader = new FileReader();
  reader.onload = processFile(files);
  reader.readAsText(files); 
}

function uploadFile(){
  var files = document.querySelector('input').files[0];
  var reader = new FileReader();
  reader.onload = processFile(files);
  reader.readAsDataURL(files); 
}

function processFile(theFile){
  return function(e) { 
    var theBytes = e.target.result; //.split('base64,')[1]; 
    document.getElementById('file').innerText = theBytes;
  }
}