JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="myFile" multiple="" />
<div id="filesize"></div>

JavaScript

$('#myFile').bind('change', function() {

  //this.files[0].size gets the size of your file.
  var fz=0
  for(var i=0; i<this.files.length; i++){
    fz+=this.files[i].size
  }
  $('#filesize').text('Your file(s) size: '+fz+' bytes')
});