let regexEscape = s => s.replace(/[-/\^$*+?.()|[]{}]/g, ‘\$&’);
let instanceRegex = new RegExp(regexEscape(‘djus.recras.nl’));
window.addEventListener(‘message’, function(e) {
let origin = e.origin || e.originalEvent.origin;
if (origin.match(instanceRegex)) {
document.getElementById(‘boekNu’).style.height = e.data.iframeHeight + ‘px’;
}
});