Javascript return undefined (async)

But the function always returns undefined although:

• I asked for a return
• the function is async
• I used .onload on the function call waiting for the response

Could you please explain me what I am doing wrong? It´s the first time I work with APIs and I am pretty confused.
Thanks

check(userInput).onload;


async function check(word) {    
    $.getJSON('http://de.wiktionary.org/w/api.php?action=parse&format=json&prop=text|revid|displaytitle&callback=?&page=' + word,
    function (json) {
        try {
            let variable = json.parse.text['*'];
            let nounM = variable.search('id=\"Substantiv,_m\">');
            let nounF = variable.search('id=\"Substantiv,_f\">');
            let nounN = variable.search('id=\"Substantiv,_n\">');
            let adjective  = variable.search("href=\"#Adjektiv");
            let gender = Math.max(nounM,nounF,nounN,adjective);
            console.log(gender);
            return gender;
        } catch (e) {
            console.log(e);
            return undefined;
        }
    });
};