field-base.js

function fieldBase () {}

fieldBase.getParentIframe = function () {
    var iframes = window.parent.document.getElementsByTagName('iframe');
    var parent;
    Array.prototype.forEach.call(iframes, function (el, i) {
        if (fieldBase.getIframeDocument(el) === window.document) {
            parent = el;
            return;
        }
    });
    return parent;
};

fieldBase.getIframeDocument = function (iframe) {
    return iframe.contentDocument || iframe.contentWindow.document;
};