Foreach Cloud Functions for Firebase

Question

My aim is to have a make a write to the database on a particular child node of each user. Below is my attempted cloud function:

const functions = require('firebase-functions');

// The Firebase Admin SDK to access the Firebase Realtime Database. 
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

    const  ref = admin.database().ref();

    exports.sync = functions.https.onRequest((req, res) => {

ref.child('users').once('value')
        .then(snap =>{
            snap.forEach(childSnap => {
                const selection = childSnap.val().selection;
                const uid = childSnap.val().uid;
                ref2 = admin.database().ref('users').child(uid).child('week1');
                ref2.set(10);
                res.send('It worked');
            });
});


});

My issue is that the code keeps returning an error message :

Error: Firebase.child failed: First argument was an invalid path: "undefined".

Is something wrong with the code or is the issue with the HTTP request capability.

Below is the JSON of my users node :

{
  "1YrpX2W2xnMPoy4YGpZcOE0xJ5g2" : {
    "email" : "muyiwaawoniyi@hotmail.com",
    "fullname" : "Muyiz",
    "selection" : [ 1, 2, 3, 4, 5, 6 ],
    "teamname" : "Donawon",
    "total" : 12,
    "uid" : "1YrpX2W2xnMPoy4YGpZcOE0xJ5g2" ,
    "userName" : "muyiwaawoniyi@hotmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 12,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "6K9rQiZQ3jaV38WWtDbNwxhqIwc2" : {
    "email" : "dami.etomi@gmail.com",
    "fullname" : "Dami Etomi",
    "selection" : 0,
    "teamname" : "Bayern Neverlosin'",
    "total" : 0,
    "userName" : "dami.etomi@gmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "9OgN4HyMtARaQEQV1mKQ5lyE1992" : {
    "email" : "jonathan10majin@gmail.com",
    "fullname" : "Jonathan Majin",
    "selection" : [ 40, 8, 10, 24, 18, 34 ],
    "teamname" : "Chad fc",
    "total" : 0,
    "userName" : "jonathan10majin@gmail.com",
    "uid": "9OgN4HyMtARaQEQV1mKQ5lyE1992",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 8,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "AGVZAUye5ZbZgvwCOpMeDkoOsEU2" : {
    "email" : "iheme.emeka@gmail.com",
    "fullname" : "Emeka Iheme",
    "selection" : 0,
    "teamname" : "Young Money",
    "total" : 0,
    "userName" : "iheme.emeka@gmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "Dh3Pz0qfTuQoOJhVILtEOnhToeT2" : {
    "email" : "femi0911@yahoo.com",
    "fullname" : "Femi Awoniyi",
    "selection" : [ 6, 33, 4, 14, 26, 50 ],
    "teamname" : "Fab FC",
    "total" : 0,
    "userName" : "femi0911@yahoo.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "EIppyMZlxZdis74yGKySU1YuEX53" : {
    "email" : "tayomichael@hotmail.com",
    "fullname" : "Tayo Olasanoye",
    "selection" : [ 3, 2, 1, 4, 5, 6 ],
    "teamname" : "ITweetBangers",
    "total" : 0,
    "userName" : "tayomichael@hotmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "FBi91g1Ms4OpqzdXE0bh4HXlOGm1" : {
    "email" : "olumidewilliams@hotmail.com",
    "fullname" : "Olumide Williams",
    "selection" : [ 20, 4, 54, 11, 53, 26 ],
    "teamname" : "Chad FC",
    "total" : 0,
    "userName" : "olumidewilliams@hotmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "KTpENH8F4GMdpYYJjdnEEvYAfsy2" : {
    "email" : "kmayanwale@yahoo.com",
    "fullname" : "Kola Ayanwale",
    "selection" : [ 5, 49, 56, 21, 29, 8 ],
    "teamname" : "Chef Olatimo",
    "total" : 16,
    "userName" : "kmayanwale@yahoo.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 16,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "RptITpORoiQlXwLYVNlzX9KmRNW2" : {
    "email" : "lemarchris@yahoo.co.uk",
    "fullname" : "Lemachi Chris-Asoluka",
    "selection" : [ 45, 40, 56, 18, 6, 9 ],
    "teamname" : "Arsene' Nah",
    "total" : 36,
    "userName" : "lemarchris@yahoo.co.uk",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 33,
    "week7" : 0,
    "week8" : 0,
    "week9" : 3
  },
  "UMFabxncKoZ6XcHpPQYZHizJ7Yr1" : {
    "email" : "awoniyideji@yahoo.com",
    "fullname" : "Deji Awoniyi",
    "selection" : [ 6, 40, 1, 15, 44, 11 ],
    "teamname" : "Deji's troops",
    "total" : 26,
    "userName" : "awoniyideji@yahoo.com",
    "week1" : 23,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 10,
    "week8" : 13,
    "week9" : 8
  },
  "WKfJhqpMiOdOQEJzKalL9JWhHR83" : {
    "email" : "ane.etomi@gmail.com",
    "fullname" : "Ané Etomi",
    "selection" : [ 4, 29, 31, 43, 41, 21 ],
    "teamname" : "MAné Event",
    "total" : 57,
    "userName" : "ane.etomi@gmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "i5oe4taDt8dehsWsy5AXCCa4QTG3" : {
    "email" : "fatekoja@gmail.com",
    "fullname" : "Fisayo Atekoja",
    "selection" : [ 14, 33, 4, 8, 40, 1 ],
    "teamname" : "Special Juan FC",
    "total" : 0,
    "userName" : "fatekoja@gmail.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "j8TYqi7toPP7cfy2y9go6PQbaNI2" : {
    "email" : "r7@yahoo.com",
    "fullname" : "err seven",
    "selection" : 0,
    "teamname" : "er",
    "total" : 0,
    "userName" : "r7@yahoo.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "mjEUeytEydRmpMu2OIPHqfz0Ex03" : {
    "email" : "jaiyeola03@hotmail.co.uk",
    "fullname" : "El Tigre",
    "selection" : [ 8, 19, 23, 44, 29, 9 ],
    "teamname" : "FC OBUNIKWE",
    "total" : 0,
    "userName" : "jaiyeola03@hotmail.co.uk",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "wdSm2CuC9WPPtDXoCcpwwcwrpU22" : {
    "email" : "foogunlana@yahoo.com",
    "fullname" : "Bo",
    "selection" : 0,
    "teamname" : "Supadej",
    "total" : 0,
    "userName" : "foogunlana@yahoo.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  },
  "zz9IVV597iZWxPbOpvNoFBpSqlf1" : {
    "email" : "nnamdi.ikeazor@yahoo.com",
    "fullname" : "Nnamdi Ikeazor",
    "selection" : [ 4, 5, 6, 29, 40, 41 ],
    "teamname" : "Pogbance FC",
    "total" : 0,
    "userName" : "nnamdi.ikeazor@yahoo.com",
    "week1" : 0,
    "week10" : 0,
    "week11" : 0,
    "week12" : 0,
    "week2" : 0,
    "week3" : 0,
    "week4" : 0,
    "week5" : 0,
    "week6" : 0,
    "week7" : 0,
    "week8" : 0,
    "week9" : 0
  }
}

Answer 1

This value is likely coming back undefined:

const uid = childSnap.val().uid;

If it's undefined, that would cause this to fail because you passed an undefined value to child():

admin.database().ref('users').child(uid)

So, check to make sure you're getting the values you expect by logging them.