Firebase function not returning value

Question

I am trying to do some operations using oncall function where, I am first checking if user exists in database, then if he exists then do some operations like updating the DB. well the function is working as expected and updating the DB as well, but its returning response as null so I can't tell what exactly happening.

exports.populateCart = functions.https.onCall((data, context) => {
    const user = context.auth.uid;

    DB.collection('users').doc(user).get().then((usr_resp) => {
        if (usr_resp.exists) {
            DB.collection('products').doc(data.productKey).get().then((prod_res) => {
                DB.collection('carts').add({
                    //some data
                }).then(() => {
                    return {
                        response: 'CART UPDATED'
                    };
                }).catch((carts_err) => {
                    return {
                        response: 'ERROR WHILE POPULATING CART : ' + carts_err
                    }
                });
            }).catch((prod_err) => {
                return {
                    response: 'ERROR WHILE FETCHING PRODUCT : ' + prod_err
                };
            });
        }
        else {
            return {
                response: 'USER DOES NOT EXISTS OR USER NOT AUTHENTICATED'
            };
        }
    }).catch((usr_err) => {
        return {
            response: 'ERROR WHILE FETCHING USER : ' + usr_err
        };
    });
});

There is one more similar question on stack overflow, where the solution is proposed to use,

reject(err);
resolve(lists);

methods, Please if the solution is using above methods, then please explain the use cases of this method in detail and if there are more methods like this please explain them as well.

Firebase function not returning data

Answer 1

You are not returning your promise. (see line 3 below)

exports.populateCart = functions.https.onCall((data, context) => {
    const user = context.auth.uid;
    // add return here before Db.collection()
    return DB.collection('users').doc(user).get().then((usr_resp) => { 
        if (usr_resp.exists) {
            return DB.collection('products').doc(data.productKey).get().then((prod_res) => {
              return  DB.collection('carts').add({
                    //some data
                }).then(() => {
                    return {
                        response: 'CART UPDATED'
                    };
                }).catch((carts_err) => {
                    return {
                        response: 'ERROR WHILE POPULATING CART : ' + carts_err
                    }
                });
            }).catch((prod_err) => {
                return {
                    response: 'ERROR WHILE FETCHING PRODUCT : ' + prod_err
                };
            });
        }
        else {
            return {
                response: 'USER DOES NOT EXISTS OR USER NOT AUTHENTICATED'
            };
        }
    }).catch((usr_err) => {
        return {
            response: 'ERROR WHILE FETCHING USER : ' + usr_err
        };
    });
});

Answer 2

You need to return every promise that you are running:

exports.populateCart = functions.https.onCall((data, context) => {
    const user = context.auth.uid;

    return DB.collection('users').doc(user).get().then((usr_resp) => {
        if (usr_resp.exists) {
            return DB.collection('products').doc(data.productKey).get().then((prod_res) => {
                return DB.collection('carts').add({
                    //some data
                }).then(() => {
                    return {
                        response: 'CART UPDATED'
                    };
                }).catch((carts_err) => {
                    return {
                        response: 'ERROR WHILE POPULATING CART : ' + carts_err
                    }
                });
            }).catch((prod_err) => {
                return {
                    response: 'ERROR WHILE FETCHING PRODUCT : ' + prod_err
                };
            });
        }
        else {
            return {
                response: 'USER DOES NOT EXISTS OR USER NOT AUTHENTICATED'
            };
        }
    }).catch((usr_err) => {
        return {
            response: 'ERROR WHILE FETCHING USER : ' + usr_err
        };
    });
});

Try copying the above code. Also using async functions will make it look better like this:

exports.populateCart = functions.https.onCall(async (data, context) => {
  try {
    const usr_resp = await DB.collection('users').doc(user).get()
    if (!usr_resp.exists) throw new Error("USER DOES NOT EXISTS OR USER NOT AUTHENTICATED")
    const prod_res = await DB.collection('products').doc(data.productKey).get()
    await DB.collection('carts').add({ . someData . })
    return { response: "Cart Updated" }
  } catch (e) {
    console.log(e)
    return { response: e }
  }
});