// Because we want to access DOM nodes,
// we initialize our script at page load.
window.addEventListener( 'load', function () {

  // These variables are used to store the form data
  const text = document.getElementById( "theText" );
  const file = {
        dom    : document.getElementById( "theFile" ),
        binary : null
      };

  // Use the FileReader API to access file content
  const reader = new FileReader();

  // Because FileReader is asynchronous, store its
  // result when it finishes to read the file
  reader.addEventListener( "load", function () {
    file.binary = reader.result;
  } );

  // At page load, if a file is already selected, read it.
  if( file.dom.files[0] ) {
    reader.readAsBinaryString( file.dom.files[0] );
  }

  // If not, read the file once the user selects it.
  file.dom.addEventListener( "change", function () {
    if( reader.readyState === FileReader.LOADING ) {
      reader.abort();
    }

    reader.readAsBinaryString( file.dom.files[0] );
  } );

  // sendData is our main function
  function sendData() {
    // If there is a selected file, wait it is read
    // If there is not, delay the execution of the function
    if( !file.binary && file.dom.files.length > 0 ) {
      setTimeout( sendData, 10 );
      return;
    }

    // To construct our multipart form data request,
    // We need an XMLHttpRequest instance
    const XHR = new XMLHttpRequest();

    // We need a separator to define each part of the request
    const boundary = "blob";

    // Store our body request in a string.
    let data = "";

    // So, if the user has selected a file
    if ( file.dom.files[0] ) {
      // Start a new part in our body's request
      data += "--" + boundary + "\r\n";

      // Describe it as form data
      data += 'content-disposition: form-data; '
      // Define the name of the form data
            + 'name="'         + file.dom.name          + '"; '
      // Provide the real name of the file
            + 'filename="'     + file.dom.files[0].name + '"\r\n';
      // And the MIME type of the file
      data += 'Content-Type: ' + file.dom.files[0].type + '\r\n';

      // There's a blank line between the metadata and the data
      data += '\r\n';

      // Append the binary data to our body's request
      data += file.binary + '\r\n';
    }

    // Text data is simpler
    // Start a new part in our body's request
    data += "--" + boundary + "\r\n";

    // Say it's form data, and name it
    data += 'content-disposition: form-data; name="' + text.name + '"\r\n';
    // There's a blank line between the metadata and the data
    data += '\r\n';

    // Append the text data to our body's request
    data += text.value + "\r\n";

    // Once we are done, "close" the body's request
    data += "--" + boundary + "--";

    // Define what happens on successful data submission
    XHR.addEventListener( 'load', function( event ) {
      alert( 'Yeah! Data sent and response loaded.' );
    } );

    // Define what happens in case of error
    XHR.addEventListener( 'error', function( event ) {
      alert( 'Oops! Something went wrong.' );
    } );

    // Set up our request
    XHR.open( 'POST', 'https://example.com/cors.php' );

    // Add the required HTTP header to handle a multipart form data POST request
    XHR.setRequestHeader( 'Content-Type','multipart/form-data; boundary=' + boundary );

    // And finally, send our data.
    XHR.send( data );
  }

  // Access our form...
  const form = document.getElementById( "theForm" );

  // ...to take over the submit event
  form.addEventListener( 'submit', function ( event ) {
    event.preventDefault();
    sendData();
  } );
} );