Cloud Functions for Firebase - Error serializing return value:

Question

I have a Cloud Function used to cross reference two lists and find values that match each other across the lists. The function seems to be working properly, however in the logs I keep seeing this Error serializing return value: TypeError: Converting circular structure to JSON . Here is the function...

exports.crossReferenceContacts = functions.database.ref('/cross-ref-contacts/{userId}').onWrite(event => {

    if (event.data.previous.exists()) {
        return null;
    }

    const userContacts = event.data.val();
    const completionRef = event.data.adminRef.root.child('completed-cross-ref').child(userId);
    const removalRef = event.data.ref;

    var contactsVerifiedOnDatabase ={};
    var matchedContacts= {};


    var verifiedNumsRef = event.data.adminRef.root.child('verified-phone-numbers');
    return verifiedNumsRef.once('value', function(snapshot) {

        contactsVerifiedOnDatabase = snapshot.val();

        for (key in userContacts) {
            //checks if a value for this key exists in `contactsVerifiedOnDatabase`
            //if key dioes exist then add the key:value pair to matchedContacts
        };

        removalRef.set(null); //remove the data at the node that triggered this onWrite function
        completionRef.set(matchedContacts); //write the new data to the completion-node

    });

});

I tried putting return in front of completionRef.set(matchedContacts); but that still gives me the error. Not sure what I am doing wrong and how to rid the error. Thanks for your help

Answer 1

I was having the exact same issue when returning multiple promises that were transactions on the Firebase database. At first I was calling:

return Promise.all(promises);

My promises object is an array that I'm using where I'm pushing all jobs that need to be executed by calling promises.push(<add job here>). I guess that this is an effective way of executing the jobs since now the jobs will run in parallel.

The cloud function worked but I was getting the exact same error you describe.

But, as Michael Bleigh suggested on his comment, adding then fixed the issue and I am no longer seeing that error:

return Promise.all(promises).then(() => {
  return true;
}).catch(er => {
  console.error('...', er);
});

If that doesn't fix your issue, maybe you need to convert your circular object to a JSON format. An example is written here, but I haven't tried that: https://stackoverflow.com/a/42950571/658323 (it's using the circular-json library).

UPDATE December 2017: It appears that in the newest Cloud Functions version, a cloud function will expect a return value (either a Promise or a value), so return; will cause the following error: Function returned undefined, expected Promise or value although the function will be executed. Therefore when you don't return a promise and you want the cloud function to finish, you can return a random value, e.g. return true;

Answer 2

Try:

return verifiedNumsRef.once('value').then(function(snapshot) {
    contactsVerifiedOnDatabase = snapshot.val();

    for (key in userContacts) {
        //checks if a value for this key exists in `contactsVerifiedOnDatabase`
        //if key dioes exist then add the key:value pair to matchedContacts
    };

    return Promise.all([
      removalRef.set(null), //remove the data at the node that triggered this onWrite function
      completionRef.set(matchedContacts)
    ]).then(_ => true);
});

Answer 3

I had the same error output with a pretty similar setup and couldn't figure out how to get rid of this error. I'm not totally sure if every essence has been captured by the previous answers so I'm leaving you my solution, maybe it helps you.

Originally my code looked like this:

return emergencyNotificationInformation.once('value', (data) => {
    ...
    return;
});

But after adding then and catch the error did go away.

return emergencyNotificationInformation.once('value')
    .then((data) => {
        ...
        return;
    })
    .catch((error) => {
        ...
        return:
    });
}

Answer 4

We fixed a similar issue with the same error by returning Promise.resolve() at the bottom of the chain, e.g.:

return event.data.ref.parent.child('subject').once('value')
    .then(snapshot => {
        console.log(snapshot.val());
        Promise.resolve();
    }).catch(error => {
        console.error(error.toString());
    });