var proto = Object.create(HTMLElement.prototype);
proto.jsonpcallback = function(result) {
    //...
}

proto.attachedCallback = function() {
    var requestURL = "https://www.openthesaurus.de/synonyme/search?q=";
    requestURL += this.getAttribute("word").substr(1, this.getAttribute("word").length - 2);
    requestURL += "&format=application/json&callback=proto.jsonpcallback";

    var script = document.createElement('script');
    script.type = "text/javascript";
    script.src = requestURL;
    document.body.appendChild(script);
};
var XComponent = document.registerElement('synonym-list', {
    prototype: proto
});