JSFiddle - React, Tailwind, and code Playground

HTML

<html>

  <head>
    <style>
      .fileCheck {
        display: block
      }
    </style>
  </head>

  <body>
    <input type="checkbox" class="fileCheck" fileid="1">
    <input type="checkbox" class="fileCheck" fileid="2">
    <input type="checkbox" class="fileCheck" fileid="3">
    <input type="checkbox" class="fileCheck" fileid="4">
    <input type="button" value="Send" onclick="send();">
    <div id="output_string"></div>
    <div id="output_array"></div>
  </body>

</html>

JavaScript

function send() {
	var selectedFiles = [];
  $('.fileCheck:checked').each(function() { selectedFiles.push($(this).attr('fileid')); });
  var stringArray = JSON.stringify(selectedFiles);
  $('#output_string').html(stringArray);
  
  var arrayAgain = JSON.parse(stringArray);
  var output_from_array = '';
  for (var i in arrayAgain) {
  	output_from_array += arrayAgain[i] + '<br>';
  }
  $('#output_array').html(output_from_array);
}