getFullname.js

function checkProperties(obj, email) {
    let fullname = "";

    for (var key in obj) {
        if (obj[key] !== null && obj[key] != "")
            fullname = obj[key];
            break;
    }

    if (fullname != "") {
        console.log(fullname)
    } else {
        const regExp = /^([^@]+)/g;
        fullname = email.match(regExp);
        console.log(fullname[0]);
    }
}

let obj = {
    familyName: null, 
    givenName: null, 
    middleName: null, 
    namePrefix: null, 
    nameSuffix: null, 
    nickname: null}

let email = "[email protected]";

checkProperties(obj, email)