Perform Firestore query in a JavaScript function

Question

I want to perform a Firestore query in a JavaScript function, but I'm having some difficulties with promises.

Let's say I want to get the document ID from a user. So I have created this JavaScript function:

function getUid(email) {
    db.collection("users").where("email", "==", email)
    .get()
    .then(function(querySnapshot) {
        querySnapshot.forEach(function(doc) {
            return doc.id;
        });
    })
    .catch(function(error) {
        return error;
    });
}

Now when I call the function res.send(getUid("[email protected]")), it returns undefined.

Which is the correct syntax to wait until the Firestore query finsished?

res.send is just for sending a response in Google cloud functions. — Leonard Niehaus
– Leonard Niehaus, Commented May 18, 2019 at 14:19

KeroppiMomo · Accepted Answer · 2019-05-18 14:54:15Z

5

get() is an async function, so you need to wrap it into an async function. Also, you are not returning anything from the getUid function - you are just returning inside a forEach parameter. If you want to get all id from the snapshot, you can use the map function.

async function getUids(email) {
    const db = admin.firestore();
    const querySnapshot = await db.collection("users").where("email", "==", email).get();
    const uids = querySnapshot.docs.map((doc) => { return doc.id });
    return uids;
}

exports.yourFunction = functions.http.onRequest(async (req, res) => {
    const email = // ...
    res.send(await getUids(email));
});

answered May 18, 2019 at 14:54

KeroppiMomo

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3 Comments

Leonard Niehaus Over a year ago

Thanks this really helped me! But how do I catch errors with promises?

KeroppiMomo Over a year ago

If you're using await, you can wrap it into a try-catch statement. If you're using Promise.then, you can add a .catch handler after it.

Leonard Niehaus Over a year ago

Thanks! You helped me how to figure out promises.

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