getname2.js

function getName(event){
    var queryString = event.queryString;
    if(queryString && queryString.lastName){
        return queryString.lastName;
    }

    var body = JSON.parse(event.body);

    if(body && body.firstName){
        return body.firstName;
    }
        
    return "bad request";
}

var e1 = {
    "queryString": {
        "lastName": "Tanaka"
    },
    "body": null
};

var e2 = {
    "queryString": null,
    "body": '{"firstName": "Taro"}'
};

console.log(getName(e1));
console.log(getName(e2));