Promises and async/await combination, ideal way to wait for asynchronous operations?

Since your getDocument code needs to wait for an asynchronous operation that doesn't provide a Promise interface, getDocument shouldn't be an async function, because you need to create the promise manually. (And the callback you give to a non-promise-focussed function should almost never be an async function.)

function getDocument(query){
    return new Promise((resolve,reject) =>{
        MongoClient.connect(url, function(err, db) {
            if (err) {
                // Reject, don't throw
                reject(err);
                return;
            }
            console.log(query);
            db.collection("users").find(query).toArray(function(err, result) {
                if (err) {
                    // Reject, don't throw
                    reject(err);
                    return;
                }
                console.log(result);
                db.close();
                resolve(result);
            });
        });
    });
}

Alternately, use promise-enabled versions of MongoClient.connect and db.collection("users").find. MongoDB has those available in its JavaScript API now (I'm afraid I don't have the details). Then you'd use an async function with await, something like this (according to this blog post):

// BE SURE TO DOUBLE-CHECK THE DETAILS
async function getDocument(query){
    const db = await MongoClient.connect(url);
    const await result = db.collection("users").find(query).toArray();
    console.log(result);
    await db.close(); // No idea whether you need `await` here or not
    return result;
}