Sending iframe height via JS to parent document.

send-iframe-height.js

let get_document_height = function () {
  var body = document.body,
      html = document.documentElement;

  var height = Math.max( body.scrollHeight, body.offsetHeight, html.clientHeight, html.scrollHeight, html.offsetHeight );
  return height;
}

let send_document_height = function () {
  var height = get_document_height();
  parent.postMessage( JSON.stringify( {'frame_height': height} ), '*' );
}

if ( window.addEventListener ) {
  window.addEventListener( 'load', send_document_height, false );
} else if ( window.attachEvent ) { // ie8
  window.attachEvent('onload', send_document_height );
}